Switch liveRatio-related log messages to DEBUG

ninja-patch by Aleksey Yeschenko for CASSANDRA-7467
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/Memtable.java
Merge branch 'cassandra-2.1.0' into cassandra-2.1