Facilitator: remember client registrations

The flash proxy facilitator forgets a client registration after giving it to a proxy. If the client wants another proxy, it has to register again. This strains the assumption that (covert, low-bandwidth) client registrations have to be done infrequently. Rather, the facilitator should make an educated guess as to how long a client address still needs service. Ideally, a client need register only once per Tor session.