Lock acquisition in dropDatabase cmd can throw and leave the database with drop-pending set to true

The dropDatabase cmd is getting stalled on lock acquisition here after waiting for replication of the dropCollection ops, before executing the dropDb logic that can proceed once the collections are gone. We have several Guards in the dropDatabase logic to unset the drop-pending flag if there are any errors, but that lock acquisition is not guarded, the previous guard being released here and a new guard not set until afterwards here, creating a window of opportunity. Lock acquisition can throw now.