remove the index entry on oldColumn when new column is a tombstone
patch by jbellis; reviewed by Sam Tunnicliffe for CASSANDRA-5395
avoid duplicate index entries ind PrecompactedRow and ParallelCompactionIterable
patch by jbellis; reviewed by Sam Tunnicliffe for CASSANDRA-5395
Ensure that PerRowSecondaryIndex updates see the most recentvalues
patch by Sam Tunnicliffe; reviewed by jbellis for CASSANDRA-5397

Conflicts:
	CHANGES.txt
Ensure that PerRowSecondaryIndex is notified of row-level deletes
patch by Sam Tunnicliffe; reviewed by jbellis for CASSANDRA-5445

Conflicts:
	CHANGES.txt