Handle negative timestamps in writetime method

patch by slebresne; reviewed by thobbs for CASSANDRA-8139
Merge branch 'cassandra-2.0' into cassandra-2.1

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

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