This makes a whole lot of our code simpler, and makes it so that
CircMgr and DirMgr no longer need to have anything parameterized
over transports, either.

Instead of boxing Transport inside of ChanMgr, I've made a new
Connection trait that goes from a ChanTarget* straight to a Channel.
This lets us avoid having to box the intermediate TLS object.

[*] Actually, a copy of the information from a ChanTarget. Ick, but
I had to make a copy to avoid parameterizing
Connecter::build_channel.