Optimize Cell liveness checks and clean up Cell

patch by Aleksey Yeschenko; reviewed by Benedict Elliott Smith for
CASSANDRA-7119
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql/QueryProcessor.java
	test/unit/org/apache/cassandra/db/compaction/CompactionsPurgeTest.java