Cassandra should expose connected client state via JMX

There is currently no good way to determine or estimate how many clients are connected to a cassandra node without using netstat or (if using sync thrift server) counting threads. There is also no way to understand what state any given connection is in. People regularly come into #cassandra/cassandra-user@ and ask how to get the equivalent of a MySQL "SHOW FULL PROCESSLIST."

While I understand that feature parity with SHOW FULL PROCESSLIST/information_schema.processlist is unlikely, even a few basic metrics like "number of connected clients" or "number of active clients" would greatly help with this operational information need.