Provide channel pooling

The client libraries generated by https://github.com/googleapis/toolkit currently create a new channel for every API wrapper object, which is expensive and wasteful. For the benefit of users, we need channel pooling support so that startup time and resource usage is minimized, regardless of how many API wrapper objects are created. The alternatives require more work for users (which is undesirable) or implementing pooling support in the API wrapper layer, which doesn't make sense if grpc is a better place to implement it.