HBASE-8190 Add documentation of region merges to the book

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1461396 13f79535-47bb-0310-9956-ffa450edef68
HBASE-8163 MemStoreChunkPool: An improvement for JAVA GC when using MSLAB

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1461398 13f79535-47bb-0310-9956-ffa450edef68