storagemgr: Remove packed-refs lockfile on start

Should a process which has locked the packed-refs file crash, the transaction manager may fail to apply a log entry due to the stale lockfiles.

To prevent this, we remove both packed-refs.lock and packed-refs.new if present when initializing the transaction manager. We provide no grace period for these files as we know that no other process should be touching the repository when the transaction manager is starting.

Closes #5313 (closed)