Fix paging with SELECT DISTINCT

patch by slebresne; reviewed by thobbs for CASSANDRA-6857
Allow deleting snapshots from dropped keyspaces
patch by Lyuben Todorov; reviewed by Nick Bailey for CASSANDRA-6281
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