public static Optional<RpcCall> getCurrentCall() {
    return Optional.ofNullable(CurCall.get());
  }
Needed for features such as delayed calls. We need to be able to store the current call so that
we can complete it later or ask questions of what is supported by the current ongoing call.
@return An RpcCallContext backed by the currently ongoing call (gotten from a thread local)
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/RpcServer.java