void removeReceiver(Connection con) {
    synchronized (receivers) {
      receivers.remove(con);
    }
  }
Close all receiving threads. This is used during shutdown and is also used by a test hook that
makes us deaf to incoming messages.

@param beingSick a test hook to simulate a sick process
geode-core/src/main/java/org/apache/geode/internal/tcp/ConnectionTable.java