Automatic expiry of subtables

Typically, if the client simply disappears, then the entry in the connection table will timeout, and CLIENT_CLOSED will fire. There are some rare instances where CLIENT_CLOSED will not fire, though. One of the features of the table command was to handle exactly this case. Even if CLIENT_CLOSED does not fire, the timer will get automatically canceled when the connection expires, and so the subtable entry will stop getting updated, and will also expire on its own. So in this case, the code in CLIENT_CLOSED could be viewed as simply an optimization, cleaning up the entry as soon as we know it isn't needed, and not waiting for it to timeout.