6233
Avoid stack overflow on RangeTombstoneList insertion

patch by slebresne; reviewed by frousseau for CASSANDRA-6181
Update changelog
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
cqlsh: add SHOW SESSION <tracing-session> command

patch by Mikhail Stepura; reviewed by Aleksey Yeschenko for
CASSANDRA-6228