In client and server, each format now has a fixed, unique, known in
advance SSRC, that is applied to outgoing packets belonging to each
format.
This is needed to support SRTP/MIKEY, that require each format to have
a fixed, unique, and known in advance SSRC.
A secondary effect is that SETUP responses now always contain SSRCs of
each format, regardless of the fact that the first packet has been
produced or not (previously we needed at least one packet, from which
the SSRC was extracted).
* add BytesSent to ServerStream (to allow MediaMTX to gather bytes sent on all paths)
* improve performance
* add test
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Co-authored-by: aler9 <46489434+aler9@users.noreply.github.com>
