Stop inheriting liveRatio and liveRatioComputedAt from previous memtables

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for CASSANDRA-7796
Merge branch 'cassandra-2.0' into cassandra-2.1.0

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