SSP doesn't cache seeds forever

Patch by brandonwilliams, reviewed by jbellis for CASSANDRA-7663
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/config/DatabaseDescriptor.java
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
Don't use strict consistency when replacing

Patch by brandonwilliams, reviewed by tjake for CASSANDRA-7568