HBASE-9164 Convert List<? extends Cell> anti pattern to List<Cell> pattern

This patch also starts a refactor of the KVComparator.


git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1514400 13f79535-47bb-0310-9956-ffa450edef68
hbase-9222: Thrift DemoClient failed with error IllegalArgument(message:Row length is 0)

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1514401 13f79535-47bb-0310-9956-ffa450edef68