- history: Added a mailing list thread recalled by Nathan Freitas
- TURN description revised to account for single allocation
  per local port.
- Revised the distinction between different Tor stream usage strategies.

I moved the "One Stream per Mapping" section up and included it as part
of a series of "One stream per Local Port" variants. In that series I
started with the standardized TURN protocol, then switched gears to
Proposal 339 and a couple further iterations on its basic idea.