Allocator refactor

With new features such as dynamic reservation, persistent volume, quota, optimistic offers, it has been apparent that we need to refactor the allocator to

1. solidify the API (e.g. consolidate updateSlave and updateAvailable)
2. possibly move the offer generation to the allocator from the master
3. support for allocator modules where the API involves returning libprocess::Future

The sequence of implementation challenges for dynamic reservation master endpoints are captured in this document.