We need to set up the pion/webrtc logger to write output to the
snowflake log, otherwise the warnings we are getting from the pion
library are being lost.

Note: this requires go version 1.13 and later in order to use the
`log.Writer()` function.

This commit fixes a small error introduced in a previous commit. Servers
given by command line options weren't being added to the configuration
because we were checking for `iceServers` to be nil instead of not nil.

Modified the snowflake client to use pion/webrtc as the webrtc library.
This involved a few small changes to match function signatures as well
as several larger ones:
- OnNegotiationNeeded is no longer supported, so CreateOffer and
SetLocalDescription have been moved to a go routine called after the
other peer connection callbacks are set
- We need our own deserialize/serialize functions
- We need to use a SettingEngine in order to access the
OnICEGatheringStateChange callback

MaxBytesReader is only documented for server side reads, so we're using
a local limitedRead function instead that uses an io.LimitedReader.

Declared limits in a commented constant

This is a fix for #26348

Amounts to the same thing as currently implemented in go-webrtc

Removed raw bytes of ICE answers from broker logs and the SDP stanzas
from the client logs

Maybe we'll able to remove this in the far future.

https://bugs.torproject.org/25600#comment:14

Snowflake in Tor Browser has been hanging after surfing for a while.
(Tor reports "no running bridges".) It only began happening after commit
12922a23, which caused snowflake-client not to log to a file by
default (leaving it to log to its default stderr). What seems to be
happening is that tor doesn't read from its PT clients' stderr, leaving
a buffer to fill up that eventually causes a hang.

https://bugs.torproject.org/22782

https://snowflake-reg-test.appspot.com/ for the client (needs domain fronting)
https://snowflake-broker.bamsoftware.com/ for the proxies (no fronting)

Note that fronting to App Engine doesn't currently actually work because
of https://bugs.torproject.org/25804; this commit doesn't change that.

Cf. https://bugs.torproject.org/24875

This actually caused a nil pointer dereference panic when Accept
returned an error, because conn was nil.

From https://trac.torproject.org/projects/tor/ticket/21312#comment:33

David Fifield authored 7 years ago and Arlo Breault committed 7 years ago

This way works when the base URL lacks a path, e.g.
"http://127.0.0.1:8080".
https://bugs.torproject.org/25472

It also expects to be called with a non-empty string, which isn't
the case with this Split which always envokes the loop at least
once.  If -ice was omitted, an error was logged.

 * From https://github.com/keroserene/go-webrtc/pull/59

Rather than having it happen as a side effect of flag parsing.

Having it happen as a side effect of flag parsing caused log messages
related to handling the option (e.g. "IceServerList:") to always go to
stderr, despite the presence of a -log option. This is because the
requested log file had not been opened yet.

 - stabilize rest of WebRTCPeer
 - use uid for the datachannel label
 - store a consistent WebRTC config in the dialer