[#2308] Use SelectorProvider.open*() to open NIO channels and so remove condition when create new NIO channels.

Motivation:
At the moment we use SocketChannel.open(), ServerSocketChannel.open() and DatagramSocketChannel.open(...) within the constructor of our
NIO channels. This introduces a bottleneck if you create a lot of connections as these calls delegate to SelectorProvider.provider() which
uses synchronized internal. This change removed the bottleneck.

Modifications:
Obtain a static instance of the SelectorProvider and use SelectorProvider.openSocketChannel(), SelectorProvider.openServerSocketChannel() and
SelectorProvider.openDatagramChannel(). This eliminates the bottleneck as SelectorProvider.provider() is not called on every channel creation.

Result:
Less conditions when create new channels.

pom.xml

@@ -433,6 +433,7 @@
             <ignore>java.nio.channels.DatagramChannel</ignore>
             <ignore>java.nio.channels.MembershipKey</ignore>
             <ignore>java.net.StandardProtocolFamily</ignore>
+            <ignore>java.nio.channels.spi.SelectorProvider</ignore>
 
             <!-- Used for NIO. 2 -->
             <ignore>java.nio.channels.AsynchronousChannel</ignore>

transport/src/main/java/io/netty/channel/socket/nio/NioDatagramChannel.java

@@ -43,6 +43,7 @@ import java.nio.ByteBuffer;
 import java.nio.channels.DatagramChannel;
 import java.nio.channels.MembershipKey;
 import java.nio.channels.SelectionKey;
+import java.nio.channels.spi.SelectorProvider;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Iterator;
@@ -60,6 +61,7 @@ public final class NioDatagramChannel
         extends AbstractNioMessageChannel implements io.netty.channel.socket.DatagramChannel {
 
     private static final ChannelMetadata METADATA = new ChannelMetadata(true);
+    private static final SelectorProvider SELECTOR_PROVIDER = SelectorProvider.provider();
 
     private final DatagramChannelConfig config;
     private final Map<InetAddress, List<MembershipKey>> memberships =
@@ -69,7 +71,13 @@ public final class NioDatagramChannel
 
     private static DatagramChannel newSocket() {
         try {
-            return DatagramChannel.open();
+            /**
+             *  Use the {@link SelectorProvider} to open {@link SocketChannel} and so remove condition in
+             *  {@link SelectorProvider#provider()} which is called by each DatagramChannel.open() otherwise.
+             *
+             *  See <a href="See https://github.com/netty/netty/issues/2308">#2308</a>.
+             */
+            return SELECTOR_PROVIDER.openDatagramChannel();
         } catch (IOException e) {
             throw new ChannelException("Failed to open a socket.", e);
         }
@@ -83,7 +91,7 @@ public final class NioDatagramChannel
         checkJavaVersion();
 
         try {
-            return DatagramChannel.open(ProtocolFamilyConverter.convert(ipFamily));
+            return SELECTOR_PROVIDER.openDatagramChannel(ProtocolFamilyConverter.convert(ipFamily));
         } catch (IOException e) {
             throw new ChannelException("Failed to open a socket.", e);
         }

transport/src/main/java/io/netty/channel/socket/nio/NioServerSocketChannel.java

@@ -30,6 +30,7 @@ import java.net.SocketAddress;
 import java.nio.channels.SelectionKey;
 import java.nio.channels.ServerSocketChannel;
 import java.nio.channels.SocketChannel;
+import java.nio.channels.spi.SelectorProvider;
 import java.util.List;
 
 /**
@@ -40,12 +41,19 @@ public class NioServerSocketChannel extends AbstractNioMessageChannel
                              implements io.netty.channel.socket.ServerSocketChannel {
 
     private static final ChannelMetadata METADATA = new ChannelMetadata(false);
+    private static final SelectorProvider SELECTOR_PROVIDER = SelectorProvider.provider();
 
     private static final InternalLogger logger = InternalLoggerFactory.getInstance(NioServerSocketChannel.class);
 
     private static ServerSocketChannel newSocket() {
         try {
-            return ServerSocketChannel.open();
+            /**
+             *  Use the {@link SelectorProvider} to open {@link SocketChannel} and so remove condition in
+             *  {@link SelectorProvider#provider()} which is called by each ServerSocketChannel.open() otherwise.
+             *
+             *  See <a href="See https://github.com/netty/netty/issues/2308">#2308</a>.
+             */
+            return SELECTOR_PROVIDER.openServerSocketChannel();
         } catch (IOException e) {
             throw new ChannelException(
                     "Failed to open a server socket.", e);

transport/src/main/java/io/netty/channel/socket/nio/NioSocketChannel.java

@@ -36,6 +36,7 @@ import java.net.SocketAddress;
 import java.nio.ByteBuffer;
 import java.nio.channels.SelectionKey;
 import java.nio.channels.SocketChannel;
+import java.nio.channels.spi.SelectorProvider;
 
 /**
  * {@link io.netty.channel.socket.SocketChannel} which uses NIO selector based implementation.
@@ -43,10 +44,17 @@ import java.nio.channels.SocketChannel;
 public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
 
     private static final ChannelMetadata METADATA = new ChannelMetadata(false);
+    private static final SelectorProvider SELECTOR_PROVIDER = SelectorProvider.provider();
 
     private static SocketChannel newSocket() {
         try {
-            return SocketChannel.open();
+            /**
+             *  Use the {@link SelectorProvider} to open {@link SocketChannel} and so remove condition in
+             *  {@link SelectorProvider#provider()} which is called by each SocketChannel.open() otherwise.
+             *
+             *  See <a href="See https://github.com/netty/netty/issues/2308">#2308</a>.
+             */
+            return SELECTOR_PROVIDER.openSocketChannel();
         } catch (IOException e) {
             throw new ChannelException("Failed to open a socket.", e);
         }