Move hints and exception count to o.a.c.metrics

patch by yukim; reviewed by jbellis for CASSANDRA-6017
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	src/java/org/apache/cassandra/service/StorageProxy.java
Fix NPE in nodetool on booting instances.
Patch by Mikhail Stepura, reviewed by brandonwilliams for CASSANDRA-5968
Merge branch 'cassandra-1.2' into cassandra-2.0