Handle empty CFs in Memtable#maybeUpdateLiveRatio()

patch by Christian Spriegel; reviewed by Aleksey Yeschenko for
CASSANDRA-7401
Merge branch 'cassandra-2.0' into cassandra-2.1

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