Log unclean client disconnect excs at DEBUG

Patch by Graham Sanderson; reviewed by Tyler Hobbs for CASSANDRA-7849
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/transport/Message.java
	src/java/org/apache/cassandra/transport/messages/ErrorMessage.java
Merge branch 'cassandra-2.1' into trunk