Create an integration test to ensure that streamed grpc endpoints cleanup after themselves

Create an integration test to ensure that https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/158 is doing what it should.

To stop this happening in future:

https://prometheus.gitlab.com/graph?g0.range_input=12h&g0.end_input=2017-04-13+20%3A15&g0.expr=sum(grpc_server_started_total%7Bjob%3D%22gitaly-production%22%7D)+-+sum(grpc_server_handled_total%7Bjob%3D%22gitaly-production%22%7D)&g0.tab=0

added Gitaly label

mentioned in merge request !160 (merged)

closed via merge request !160 (merged)

mentioned in commit tmaczukin/gitlab-workhorse@1c95f240