Fix partition and range deletes not triggering flush

patch by benedict; reviewed by slebresne for CASSANDRA-6655
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/AtomicSortedColumns.java
	src/java/org/apache/cassandra/db/DeletionInfo.java