Handle nil stats getter in collect stats

It is possible to call GetStats() after the peer connection is closed.
The statsGetter gets cleared when peer connection is closed. That is
causing a panic when RtpReceiver.collectStats runs.

Add interface for getting media-playout stats

Do not create receiver for ealy media in offerer

For offerer, if the remote side sends early media
before the remote description (answer) is received,
the undeclared SSRC processor can create a receiver
and that receiver could be left dangling as
transceiver `mid` is not updated from remote
description answer.

Still leaving the simulcast probe path and only
avoiding creating a receiver for non-simulcast path.

Add a flag `handleUndeclaredSSRCWithoutAnswer` to control handling
of early media without SDP answer for non-simulcast tracks.
The default behaviour is to not process early media without SDP answer.

 Update module github.com/pion/ice/v2 to v2.3.38

H265 reader & writer

H265 raeder & writer

Updated Test_TrackLocalStatic_Padding test

Use custom packetizer in Test_TrackLocalStatic_Padding to verify that
padding added in different ways works.

Add methods for tweaking sctp cc to settingending

Relate: pion/sctp#354

Fix MIME type comparisons to be case-insensitive

Pass Configured MTU into SCTP

Before pion/sctp would always do 1228

Added SCTPTransport.BufferedAmount

It simplifies tracking amount of unsent data for all data channels,
including ones in closing state. Especially useful for IoT apps
which use multiple data channels and have to monitor memory usage.