Always merge ranges owned by a single node in DES

Patch by Ala' Alkhaldi; review by Tyler Hobbs for CASSANDRA-6930
Add cqlsh completion for IF [NOT] EXISTS in CREATE/DROP USER

Patch by Michał Michalski; review by Tyler Hobbs for CASSANDRA-7611
Add unordered PITR test

Patch by Ala' Alkhaldi, reviewed by brandonwilliams for CASSANDRA-6905
fix test for 2.0
update CL archive comments post-6905
7601-2.0