HBASE-7239. Introduces chunked reading for large cellblocks

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1469654 13f79535-47bb-0310-9956-ffa450edef68
HBASE-8264 expose the number of seen KVs from StoreScanner

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1469668 13f79535-47bb-0310-9956-ffa450edef68