Now callers cannot call Write without there being a DataChannel to write
to. This lets us remove the internal buffer and checks for transport ==
nil.

Don't set internal fields like writePipe, transport, and pc to nil when
closing; just close them and let them return errors if further calls are
made on them.

There's now a constant DataChannelTimeout that's separate from
SnowflakeTimeout (the latter is what checkForStaleness uses). Now we can
set DataChannel timeout to a lower value, to quickly dispose of
unconnectable proxies, while still keeping the threshold for detecting
the failure of a once-working proxy at 30 seconds.

https://bugs.torproject.org/33897