Allow Tor2Web mode to specify its own Rendezvous Point

Virgil from tor2web is interested in implementing a feature of tor2web mode that will allow Tor to specify a specific node as the rendezvous point of an HS circuit.

The idea here is that if tor2web specifies itself as the RP, the latency of RP establishment during HS circuit building will decrease. (I don't know if this is true or whether the speedup will be measurable, but it's worth trying).

My idea for implementing this feature is to introduce a torrc option (temp name Tor2webRendezvousPoints) that acts similar to ExitNodes etc., and forces Tor to select one of the specified RPs as the RP of all HS circuits.

I'm calling this a tor2web feature since I don't see much use for non-tor2web clients.