Add notification of changed keys

[ecton]

Redis has several blocking features that would be good additions to BonsaiDb. They all are based on the basic need of being able to be notified when a key is changed.

An example operation is BZMPOP (relevant to #154), which allows a client to block up to an optional timeout until a key is available in the sorted set. This type of blocking operation is common in Redis and allows building worker queues easily.