Lock recovery

[denismo]

If a client system which took a look on file dies no other system will be able to lock its files. Worse, if the system restarts the in-memory lockId will be gone and the system itself won't be able to unlock.

Need to implement lock recovery - if the system is identified to be dead the locks should be released. It can be done by some other system, based on "last client check-in" time.

1. The clients can check-in every now and then recording the time in some object.
2. The other clients can read the check-in object in case of lock being held for long time, and compare it with the lock time (needs to be added as well). If the time is too far from the expected check-in another client can declare the other system dead and release its locks. Other clients would then only check the system status.
3. Also depends on LockManager resiliance #41 to implement better error handling to handle situations when the client simply timed-out - if it is unable to lock/unlock on Dynamo it should not break the client. Unlock should succeed in most cases.