HBASE-9250 Make sure lease time isn't ever negative.

Sometimes nextLease.getDelay can return a negative number.  Trying to
sleep for a negative time thows an uncaught exception bringing down the
whole regionserver.

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.95@1514898 13f79535-47bb-0310-9956-ffa450edef68