First draft of update transitions independent of rings
Update RingGroupUpdateTransitionFunctionImpl and its test to be able to consider
hosts independently.
Add test case to TestRingGroupUpdateTransitionFunctionImpl
Optimize performance of RingGroupUpdateTransitionFunctionImpl by not computing
fully serving status for each host independently.
Fix comment typo
Fix RingGroupUpdateTransitionFunctionImpl caching
Optimize performance of ring assignment by caching assignments across hosts
Improve safety of RingGroupUpdateTransitionFunctionImpl
Fix TestRingGroupUpdateTransitionFunctionImpl NPEs