It seems that bad things happen when two gRPC stubs share one gRPC
channel so let's stop doing that. The downside of this is that we
create more gRPC connections; one per stub.

Closes gitaly#174

Rails auto-load (a development feature) can end up resetting instance
variables on classes. This breaks Gitlab::GitalyClient, which uses
instance variables to keep global hashes to look up channels and
addresses. This change adds code that regenerates the hashes if they
suddenly become nil.

Closes gitaly#166