Add exclusive lease to mergeability check process
What does this MR do?
After some investigation at gitaly!1372 (merged), it was noticed that concurrent calls to
UserMergeToRef RPC updating a single ref could lead to an opaque fail that was being rescued at Gitaly (see gitaly!1372 (merged)). For
MergeabilityCheckService today it means 500's when trying to load the MR in parallel with another user.
The chosen solution is adding an exclusive lease at