An upcoming update to rubocop-gitlab-security added additional
violations.

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