RefDirectory: Do not unlock until after deleting loose ref

RefDirectory: Do not unlock until after deleting loose ref

Fix a potential race condition where we would remove our loose ref lock
file before deleting the loose ref itself. This race could result in the
current thread deleting a loose ref newly written by another thread.

Other callers seem to be following the correct pattern, but improve the
method naming to try to help future callers.

Change-Id: I80cebe4788edd855e15381336d980c41498fca80
Signed-off-by: Nasser Grainawi <quic_nasserg@quicinc.com>