Reduce channel max to 2047

Max number of channels has been reduced from unlimited to 2048 as of
RabbitMQ 3.7.5. This is to avoid applications that leak channels to
starve the broker resources. 2048 has been picked up scientifically:
more than 1 K channels per connection is hard to justify, so we doubled
the value to leave existing applications some room.

Java client uses 2047 as it uses channel 0 for negotiation and error
handling.

Part of rabbitmq/rabbitmq-server#1593.

Fixes #366

Author: acogoluegnes

src/main/java/com/rabbitmq/client/ConnectionFactory.java

@@ -63,8 +63,10 @@ public class ConnectionFactory implements Cloneable {
     /** Default virtual host */
     public static final String DEFAULT_VHOST = "/";
     /** Default maximum channel number;
-     *  zero for unlimited */
-    public static final int    DEFAULT_CHANNEL_MAX = 0;
+     *  2047 because it's 2048 on the server side minus channel 0,
+     *  which each connection uses for negotiation
+     *  and error communication */
+    public static final int    DEFAULT_CHANNEL_MAX = 2047;
     /** Default maximum frame size;
      *  zero means no limit */
     public static final int    DEFAULT_FRAME_MAX = 0;

src/test/java/com/rabbitmq/client/test/ChannelNumberAllocationTests.java

@@ -119,4 +119,10 @@ public int compare(Channel x, Channel y){
     assertNotNull(connection.createChannel(5));
     assertNotNull(connection.createChannel(1));
   }
+
+  @Test public void channelMaxIs2047() {
+    int channelMaxServerSide = 2048;
+    int defaultChannelCount = 1;
+    assertEquals(channelMaxServerSide - defaultChannelCount, connection.getChannelMax());
+  }
 }