@Override
  public <S, R> CompletableFuture<R> coprocessorService(Function<RpcChannel, S> stubMaker,
    ServiceCaller<S, R> callable, byte[] row) {
    return coprocessorService(stubMaker, callable, null, row);
  }
The implementation of RawAsyncTable.
<p/>
The word 'Raw' means that this is a low level class. The returned {@link CompletableFuture} will
be finished inside the rpc framework thread, which means that the callbacks registered to the
{@link CompletableFuture} will also be executed inside the rpc framework thread. So users who use
this class should not try to do time consuming tasks in the callbacks.
@since 2.0.0
@see AsyncTableImpl
hbase-client/src/main/java/org/apache/hadoop/hbase/client/RawAsyncTableImpl.java