Fix NPE when writetime() or ttl() are nested inside a fn call

Patch by Tyler Hobbs; reviewed by Benjamin Lerer for CASSANDRA-8451
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/statements/Selection.java
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql3/statements/Selection.java