HBASE-8706 Some improvement in snapshot (binlijin)

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1492398 13f79535-47bb-0310-9956-ffa450edef68
HBASE-8702: Make WALEditCodec pluggable

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1492407 13f79535-47bb-0310-9956-ffa450edef68