@Override
  public <S, R> CompletableFuture<R> coprocessorService(Function<RpcChannel, S> stubMaker,
    ServiceCaller<S, R> callable, byte[] row) {
    return null;
  }
Can be overridden in UT if you only want to implement part of the methods in {@link AsyncTable}.
hbase-server/src/test/java/org/apache/hadoop/hbase/client/DummyAsyncTable.java