Fix hardcoded overhead size in commit log test.

Patch by Benedict, reviewed by brandonwilliams for CASSANDRA-6974
More aggressive waiting in KeyCacheTest

Patch by Benedict, reviewed by brandonwilliams for CASSANDRA-7167
Make StreamSession more thread safe

patch by sankalp kohli; reviewed by yukim for CASSANDRA-7092
Merge branch 'cassandra-2.0' into cassandra-2.1