Provide API for durability operations

This feature will provide a callback-oriented API for durability (aka observe-poll) operations. Considering that the logic of implementing these operations has proved somewhat more complex than intended, it would be better to centralize this in libcouchbase.

The API will provide a "durability set" which is a group of keys to be monitored, a "per-key" callback for when each key reaches its persistence/durability requirements, a "set callback" for when all keys have been endured (or have encountered a condition in which no further retries can be made)