Fix memtable flushing for indexed tables

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-6112
Fix memtable flushing for indexed tables

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-6112

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/compaction/CompactionManager.java
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	src/java/org/apache/cassandra/db/compaction/CompactionManager.java