Fix paging with SELECT DISTINCT

patch by slebresne; reviewed by thobbs for CASSANDRA-6857
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	test/unit/org/apache/cassandra/db/ColumnFamilyStoreTest.java
Allow deleting snapshots from dropped keyspaces
patch by Lyuben Todorov; reviewed by Nick Bailey for CASSANDRA-6281
merge from 2.0
Make cqlsh prompt for a password if the user doesn't enter one.

patch by J.B. Langston; reviewed by Mikhail Stepura for CASSANDRA-6902
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	bin/cqlsh