Fix skipping columns with multiple slices

patch by frousseau & slebresne; reviewed by frousseau & slebresne for CASSANDRA-6119
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	src/java/org/apache/cassandra/db/columniterator/IndexedSliceReader.java
Merge branch 'cassandra-2.0' into trunk
Fix test build
Merge branch 'cassandra-2.0' into trunk