Improve MeteredFlusher handling of MF-unaffected column families

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-6867
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/MeteredFlusher.java
Merge branch 'cassandra-2.1' into trunk