CSHARP-686: Replace RequestStart/Done with IMongoBinding.
CSHARP-700: Revised after code review.
CSHARP-686: Added IClusterBinding, INodeBinding and IConnectionBinding as specializations of IMongoBinding.
CSHARP-686: Removed binding from MongoCollection. All collections derived from a database should use the database binding.
CSHARP-686: Renamed GetNodeBinding and GetConnectionBinding to NarrowToNode and NarrowToConnection (as a test).
CSHARP-686: Separate IClusterBinding, INodeBinding and ConnectionBinding into three disjoint interfaces/classes.