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