Coverage analysis for low CL queries

patch by slebresne; reviewed by jbellis for CASSANDRA-4858
Merge branch 'cassandra-1.2' into trunk

Conflicts:
	src/java/org/apache/cassandra/service/StorageProxy.java
GPFS returns a default rack/dc as a last resort.
Patch by brandonwilliams reviewed by vijay for CASSANDRA-5133
Merge branch 'cassandra-1.2' into trunk