@Override
  public int getNumOpenConnections() {
    return allChannels.size();
  }
Fatal watermark for pending outbound bytes of a single netty channel. If the number of pending
outbound bytes exceeds this threshold, the connection will be forcibly closed so that memory
can be reclaimed. The client will have to re-establish a new connection and retry any in-flight
requests.
<p>
Note: must be higher than the high watermark, otherwise it's ignored.
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/NettyRpcServer.java