Spanner Pool enhancements

[pjenvey]

We cut a couple corners while building the Pool impl. We should cleanup the following:

• Cache the Service Account Access. We currently reload the token from disk whenever creating a Connection
• We need a way to specify the Service Account file (environment variable?) and accompanying loading of it (the new client SDK will handle this for us)
• We never implemented is_valid/has_broken
• kill any .unwrap()s

Further improvements (2019-10-22):

• Multiplex SpannerClient "connections" on top of grpc::Channel (via cloning the channel per SpannerClient)
• Not only should we "cache" the service account access (they should be singletons inside the SpannerConnectionManager), but also the grpcio Env and channels themselves
• Spanner's docs recommend pooling 100 connections because 100 is the maximum number of concurrent sessions per gRPC channel. Local testing shows syncstorage-rs handling > 100 simultaneous users/db connections from the loadtester. So we should not only maintain a pool of SpannerClient "connections" but underneath that a pool of grpc Channels, with each Channel capable of 100 "connections"