Connection pool

Starting with !2340 (merged), we're growing a third ad-hoc implementation of a connection pool. To avoid code duplication and thus potential for more errors, this MR creates a new connections.Pool interface that pools connections for us. It transparently creates either a new connection if none previously existed or alternative returns an existing one. As GRPC already handles multiplexing for us and as grpc.ClientConns can safely be used concurrently, the actual logic is quite simple.

Fixes #2933 (closed)