HBASE-6765 'Take a snapshot' interface (Jesse Yates)

Add interfaces taking a snapshot. This is in hopes of cutting down on the overhead involved in reviewing snapshots.



git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/hbase-7290@1445769 13f79535-47bb-0310-9956-ffa450edef68