Remove old ref lock files during GarbageCollect RPC

After https://gitlab.com/gitlab-org/gitlab-ce/issues/40461#note_54686824

We see issues in production where some Git or libgit2 process takes a ref lock in a repository and never releases it. This probably means the process died without being able to release the lock.

When this happens the repository is left in a broken state; the locked ref can no longer be updated. This also breaks git gc, i.e. the GarbageCollect RPC.

What I propose we do is at the start of the GarbageCollect we walk the refs directory of the repository and we delete any *.lock files we find that are older than 1 hour. We should also check for the existence of a packed-refs.lock older than 1 hour in the root.

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Stages:

• Server Implementation: Create Issue

RPC Endpoints:

• RepositoryService::GarbageCollect

Known Client Routes:

• Known client endpoints

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