Cloud Bigtable should have a gRPC channel pool

###### Copied from original issue: https://github.com/GoogleCloudPlatform/google-cloud-node/issues/2343

<a href="/garye"><img src="https://avatars2.githubusercontent.com/u/37807?s=88&v=4" height=44 width=44 align=left></a>@garye<br>May 31, 2017 7:57 PM

To avoid hitting single-channel limits, the client should leverage a channel pool.