Common library: implement chunk id set [simple] checksum definition that allows incremental checksum computation, and is commutative and associative.
Meta server: order hibernated servers by location, and replace linear search with binary search to find hibernated server by location. Use hash sets to make in last in flight chunks unique. Keep track of of last in-flight chunk deletes. Add special handling for stale chunks RPC that has chunks list, and can be used with chunks available RPC. In the later case add chunk ids to in flight chunks instead of list of deleted chunks, in order to correctly disambiguate response to chunk available RPC. Implement incremental computation of chunk server's chunk ids set checksum.