Add ability to detect unofficial openjdk builds.

Patch by brandonwilliams, reviewed by enigmacurry for CASSANDRA-7799
Forbid re-adding dropped counter columns

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-7831
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt