Move latch.countDown() into "finally" block

patch by Mikhail Stepura; reviewed by Tyler Hobbs for CASSANDRA-7275
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.0' into cassandra-2.1

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