HBASE-11185 Parallelize Snapshot operations

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1596168 13f79535-47bb-0310-9956-ffa450edef68
HBASE-11186 Improve TestExportSnapshot verifications

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1596172 13f79535-47bb-0310-9956-ffa450edef68