Track liveRatio per-memtable, not per-CF

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-6945
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/ColumnFamilyStore.java
	src/java/org/apache/cassandra/db/DataTracker.java
	src/java/org/apache/cassandra/db/Memtable.java