CellComparator perf improvement

We have multiple perf issues in 2.x versions compared to 1.x. Eg: HBASE-24754, HBASE-24637.
The pattern is clear that where ever we do more and more Cell compares, there is some degrade. In HBASE-24754, with an old KVComparator style comparator, we see much better perf for the PutSortReducer. (Again the gain is huge because of large number of compare ops that test is doing). This issue is to address and optimize compares generally in CellComparatorImpl itself.