clarify error messages for zero/multiple PKs
patch by Lyben Todorov; reviewed by jbellis for CASSANDRA-5875
increase -Xss to 256k

Patch by eevans; reviewed by jbellis for CASSANDRA-5517
Cleanup doesn't need to inspect sstables that contain only local data
patch by Tyler Hobbs; reviewed by jbellis for CASSANDRA-5722
Merge branch 'cassandra-2.0' into trunk