HBASE-12313 Redo the hfile index length optimization so cell-based rather than serialized KV key

Conflicts:
	hbase-common/src/main/java/org/apache/hadoop/hbase/CellComparator.java
	hbase-common/src/main/java/org/apache/hadoop/hbase/CellKey.java
	hbase-common/src/test/java/org/apache/hadoop/hbase/TestCellUtil.java