Set gc_grace_seconds to 7 days for system schema tables

Patch by Tyler Hobbs; review by Aleksey Yeschenko for CASSANDRA-7668
Merge branch 'cassandra-1.2' into cassandra-2.0

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

Conflicts:
	CHANGES.txt