add nodetool enablebackup/disablebackup
patch by pmargolin reviewed by dbrosius for CASSANDRA-5556
Merge branch 'cassandra-1.2' into trunk
don't create empty RowMutations in CommitLogReplayer
patch by jbellis reviewed by dbrosius for (CASSANDRA-5541)
Ensure that submitBackground enqueues at least one task
patch by Oleg Anastasyev; reviewed by jbellis for CASSANDRA-5554