Unify index update structures in IndexRegionObserver and IndexCommitter

The index updates are represented by a Collection<Pair<Mutation, byte[]>> where the byte[] member of the pair is the name of the index table. However, this map is then translated to a Multimap<HTableInterfaceReference, Mutation> when it is passed to an IndexCommitter. The code in IndexRegionObserver gets simplified and becomes more efficient in cpu utilization if IndexRegionObserver also uses a Multimap<HTableInterfaceReference, Mutation> structure to represent index updates.