Clear cell queues when marking or truncating a circuit.

Clear cell queues when marking or truncating a circuit.

At best, this patch helps us avoid sending queued relayed cells that
would get ignored during the time between when a destroy cell is
sent and when the circuit is finally freed.  At worst, it lets us
release some memory a little earlier than it would otherwise.

Fix for bug #1184.  Bugfix on 0.2.0.1-alpha.