Restore expiring->deleted (cell) compaction optimization

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-6844
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
fixed bug in CompactionManager.needsCleanup

Patch by Dmitrij Koniajev, reviewed by marcuse for CASSANDRA-6845.
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt