#226 Update to allow for use of rtcp-mux where RTP is multiplexed with RTCP

[hamptonng]

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✅ kYroL01
✅ lmangani
❌ Neil Hampton

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[hamptonng]

Not sure why, but sometimes when I run this after this change I get the following error after an RTCP message is detected:

2022/04/08 14:30:26.608643 hep.go:92: ERR write udp 10.202.118.101:42832->10.202.64.185:9060: write: message too long

This doesn't happen on every run of heplify, but when it happens it is for every RTCP message and doesn't stop until I restart heplify.

[adubovikov]

@kYroL01 can we merge ?

[kYroL01]

@adubovikov not yet because I spotted a little bug and I need to test better. The PR is correct for that case but for default not, so I'm reverting a change that I think create the problem, do another full test and then we can merge