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.

Start by tracing how the client libraries generated by googleapis/toolkit create channels for each API wrapper object. Determine whether pooling belongs in the gRPC Java layer rather than the API wrapper layer, then define and test behavior for multiple wrapper objects, including reduced startup time and resource usage. The issue names no specific files or tests.

Vom Indexierungsmodell aus dem Issue-Text verfasst.