[#2656] Minimize ByteBuf.writeBytes(...) calls by pack the separator into the HttpHeaderEntity.

Motivation:
Currently we do 4 ByteBuf.writeBytes(...) calls per header line. This is can be improved.

Modification:
Introduce two new HttpHeaders methods to allow create HttpHeaderEntity which contains the separator. With this we can minimize it to 2 ByteBuf.writeBytes(...) calls per header line

Result:
Performance improvement.

codec-http/src/main/java/io/netty/handler/codec/http/HttpHeaderEntity.java

@@ -23,11 +23,25 @@ final class HttpHeaderEntity implements CharSequence {
     private final String name;
     private final int hash;
     private final byte[] bytes;
+    private final int separatorLen;
 
     public HttpHeaderEntity(String name) {
+        this(name, null);
+    }
+
+    public HttpHeaderEntity(String name, byte[] separator) {
         this.name = name;
         hash = HttpHeaders.hash(name);
-        bytes = name.getBytes(CharsetUtil.US_ASCII);
+        byte[] nameBytes = name.getBytes(CharsetUtil.US_ASCII);
+        if (separator == null) {
+            bytes = nameBytes;
+            separatorLen = 0;
+        } else {
+            separatorLen = separator.length;
+            bytes = new byte[nameBytes.length + separator.length];
+            System.arraycopy(nameBytes, 0, bytes, 0, nameBytes.length);
+            System.arraycopy(separator, 0, bytes, nameBytes.length, separator.length);
+        }
     }
 
     int hash() {
@@ -36,11 +50,14 @@ final class HttpHeaderEntity implements CharSequence {
 
     @Override
     public int length() {
-        return bytes.length;
+        return bytes.length - separatorLen;
     }
 
     @Override
     public char charAt(int index) {
+        if ((bytes.length - separatorLen) <= index) {
+            throw new IndexOutOfBoundsException();
+        }
         return (char) bytes[index];
     }
 
@@ -54,7 +71,8 @@ final class HttpHeaderEntity implements CharSequence {
         return name;
     }
 
-    void encode(ByteBuf buf) {
+    boolean encode(ByteBuf buf) {
         buf.writeBytes(bytes);
+        return separatorLen > 0;
     }
 }

codec-http/src/main/java/io/netty/handler/codec/http/HttpHeaders.java

@@ -1363,17 +1363,20 @@ public abstract class HttpHeaders implements Iterable<Map.Entry<String, String>>
 
     @SuppressWarnings("deprecation")
     static void encode(CharSequence key, CharSequence value, ByteBuf buf) {
-        encodeAscii(key, buf);
-        buf.writeBytes(HEADER_SEPERATOR);
-        encodeAscii(value, buf);
-        buf.writeBytes(CRLF);
+        if (!encodeAscii(key, buf)) {
+            buf.writeBytes(HEADER_SEPERATOR);
+        }
+        if (!encodeAscii(value, buf)) {
+            buf.writeBytes(CRLF);
+        }
     }
 
-    public static void encodeAscii(CharSequence seq, ByteBuf buf) {
+    public static boolean encodeAscii(CharSequence seq, ByteBuf buf) {
         if (seq instanceof HttpHeaderEntity) {
-            ((HttpHeaderEntity) seq).encode(buf);
+            return ((HttpHeaderEntity) seq).encode(buf);
         } else {
             encodeAscii0(seq, buf);
+            return false;
         }
     }
 
@@ -1395,6 +1398,28 @@ public abstract class HttpHeaders implements Iterable<Map.Entry<String, String>>
         return new HttpHeaderEntity(name);
     }
 
+    /**
+     * Create a new {@link CharSequence} which is optimized for reuse as {@link HttpHeaders} name.
+     * So if you have a Header name that you want to reuse you should make use of this.
+     */
+    public static CharSequence newNameEntity(String name) {
+        if (name == null) {
+            throw new NullPointerException("name");
+        }
+        return new HttpHeaderEntity(name, HEADER_SEPERATOR);
+    }
+
+    /**
+     * Create a new {@link CharSequence} which is optimized for reuse as {@link HttpHeaders} value.
+     * So if you have a Header value that you want to reuse you should make use of this.
+     */
+    public static CharSequence newValueEntity(String name) {
+        if (name == null) {
+            throw new NullPointerException("name");
+        }
+        return new HttpHeaderEntity(name, CRLF);
+    }
+
     protected HttpHeaders() { }
 
     /**