An owner that exits while its unlock cannot run leaves the key held with no record #43

An owner that exits while its unlock cannot run leaves the key held with no record. The owner DOWN handler ignores the result of the unlock and deletes the record, so if the lock connection stalls while idle, the unlock queues behind the pool's ping and is dropped after about 4 to 6 s, and if the stall ends before the 15 s ping timeout, the live session keeps the key, nothing is logged, nothing retries the unlock, and other nodes get :lock_already_taken until that connection ends. If the subscription starts again on the same node, it takes the key once more, so it stays pinned to that node, and later stops on a healthy connection still leave the key held. release/2 keeps a failed unlock and retries it. Same at 19072b4. Low: it needs a stall of several seconds with a subscription stopping during it.

Part of the review of main at 1f9d1f8: #27 (comment)