Don't shut ExpiringMap down.

Patch by Benedict, reviewed by brandonwilliams for CASSANDRA-6948
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
Fix collection element access in IF

patch by slebresne; reviewed by thobbs for CASSANDRA-6914
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/cql3/ColumnCondition.java
Allow compaction of system tables during startup
patch by Benedict Elliott Smith; reviewed by jbellis for CASSANDRA-6913
merge from 2.0
add asserts to make sure we don't call KS.open during startup
patch by Benedict Elliott Smith; reviewed by jbellis