Added safe relock function 

Required a changed in the lock function to return the lock ID instead of the unlock callback.
Apologies for the delay Maciej, and thanks for the details on your use case.

How long is your delay? Not sure exactly how you are obtaining ops from MongoDB but you might  miss some during this failover delay window?

Is there a way to use the failover built into MongoDB itself to achieve similar?

Anyway, I think a method to extend the TTL of an existing lock is useful. We can call it `touch()`. Extra param of lock ID is preferable to changing the type of first param, so I'd go with that.