Wait for data channel OnOpen before returning from NewWebRTCPeer.
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