Users sometimes click on the button multiple times.

If a connetion is slow to get established, this would cause it to
be pushed twice, the first time before it is complete.  Thanks to
Roman Koval who noticed the issue.

Apparently, it is possible for an ICE candidate to be generated
before onnegotiationneeded triggers.  Avoid losing a candidate
in that case.

Probably not necessary, but I'm supersticious.

We already send NACKs when a packet is missing.  Under high packet loss,
however, the recovery packet might get lost two.  Forward receiver NACKs
to the sender, but only after a delay and after checking that the packet
has not arrived in the meantime.