pubsub client now on gRPC

[Jonathan Amsterdam]

The cloud.google.com/go/pubsub package now uses gRPC for communication. This is not a breaking change, but it may have performance implications if you create many pubsub.Clients (for example, if you create a client for each request to your server). Each client manages its own gRPC connection, which typically corresponds to a single TCP connection. You should create clients sparingly and call a client's Close method when you're finished with it (although it's not necessary to call Close when your program terminates).