add TorClientEndpoint.from_connection

[meejah]

Add a static method that can create a TorClientEndpoint given a TorControlProtocol connection, first querying the SOCKSPort(s) and giving back a usable TCP stream.

Stream-isolation should be supported (and be the default out-of-the-box), probably via kwargs that produce random authentication if not provided (?)

See also: https://tahoe-lafs.org/pipermail/tahoe-dev/2015-June/009455.html