Don't create a new ParserImpl on each parser() call (#11255)

Motivation:

ParserImpl is stateless and so we can use the same instance multiple times

Modifications:

- Make constructor private
- Return the same instance all the time

Result:

Less object creation

resolver/src/main/java/io/netty/resolver/HostsFileEntriesProvider.java

@@ -120,7 +120,7 @@ public final class HostsFileEntriesProvider {
      * @return a new {@link HostsFileEntriesProvider.Parser}
      */
     public static Parser parser() {
-        return new ParserImpl();
+        return ParserImpl.INSTANCE;
     }
 
     static final HostsFileEntriesProvider EMPTY =
@@ -164,6 +164,12 @@ public final class HostsFileEntriesProvider {
 
         private static final InternalLogger logger = InternalLoggerFactory.getInstance(Parser.class);
 
+        static final ParserImpl INSTANCE = new ParserImpl();
+
+        private ParserImpl() {
+            // singleton
+        }
+
         @Override
         public HostsFileEntriesProvider parse() throws IOException {
             return parse(locateHostsFile(), Charset.defaultCharset());