Fix periodic memtable flushing behavior with clean memtables

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-5931
Merge branch 'cassandra-2.0.0' into cassandra-2.0
Correct minor CQL3 grammar rules (USING)
Merge branch 'cassandra-1.2' into cassandra-2.0.0

Conflicts:
	src/java/org/apache/cassandra/cql3/Cql.g
Merge branch 'cassandra-2.0.0' into cassandra-2.0