Correctly handle null in conditions with TTL

patch by slebresne; reviewed by iamaleksey for CASSANDRA-6623
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql3/statements/ModificationStatement.java
	src/java/org/apache/cassandra/service/StorageProxy.java
Account for range and row tombstones in tombstone drop time histogram.

Patch by marcuse, reviewed by jbellis for CASSANDRA-6522
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/ColumnFamily.java
	src/java/org/apache/cassandra/tools/SSTableMetadataViewer.java