limithandler: add concurrency queue limit
The concurrency queue can grow unbounded. This can lead to production incidents whereby the latency of incoming requests keep growing since Gitaly can't process requests fast enough.
This change introduces a global (per RPC) queue size limit. The concurrency queue will not be able to grow beyond this limit.
addresses: https://gitlab.com/gitlab-org/gitaly/-/issues/4001
Edited by John Cai