TcpClient Pool and/or Async Usage

[zachkinstner]

Based on the latest RemoteGremlinTests results and discussion, it looks like creating a new TcpClient for each request adds about 1.5ms to the total execution time. TcpClient connections could be pooled for reuse.

Also, consider connecting asynchronously. This may help RexConnect's times in RemoteGremlinTests parallel mode. See this async example.

I'm not sure if this kind of functionality belongs in RexConnectClient. It seems like a good option to offer, at least.