@Override
          protected void doRespond(RpcResponse resp) {
            if (writeCount.incrementAndGet() >= flushAfter) {
              super.doRespond(resp);
            } else {
              channel.write(resp);
            }
          }
Test that our fatal watermark is honored, which requires artificially causing some queueing so
that pendingOutboundBytes increases.
hbase-server/src/test/java/org/apache/hadoop/hbase/ipc/TestNettyChannelWritability.java