HBASE-7703 Eventually all online snapshots fail due to Timeout at same regionserver.

Online snapshot attempts would fail due to timeout because a rowlock could not be obtained.  Prior to this a
cancellation occurred which likely grabbed the lock without cleaning it properly. The fix here is to use nice cancel
instead of interrupting cancel on failures.



git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/hbase-7290@1445866 13f79535-47bb-0310-9956-ffa450edef68