Avoid dropping packets for simulcast probes

[cptpcrd]

Description

Currently, when Pion is receiving simulcast, PeerConnection.handleIncomingSSRC reads (at least) 2 packets: one packet is used to determine the payload type, and one (or more) packets are used to determine the MID, RID, and repair stream ID. This means that the first video frame is always partially lost, which is undesirable.

To fix this:

1. Get the initial payload type from the SRTP read stream to avoid having to consume the first packet.
2. Peek (instead of reading) from the SRTP read stream to avoid having to consume the second packet (assuming that the MID/RID/repair stream ID come in right away).

(This patch relies on corresponding changes to pion/srtp which I will PR shortly and link back here.)

Reference issue

N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.09%. Comparing base (eb6e395) to head (fd2aab2).
Report is 10 commits behind head on v3.

Additional details and impacted files

@@             Coverage Diff             @@
##               v3    #2777       +/-   ##
===========================================
- Coverage   76.32%   65.09%   -11.23%     
===========================================
  Files          87       64       -23     
  Lines        9846     3100     -6746     
===========================================
- Hits         7515     2018     -5497     
+ Misses       1866      954      -912     
+ Partials      465      128      -337     

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wasm	65.09% <ø> (+0.64%)	⬆️

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[adriancable]

@cptpcrd - does your issue #1 (consuming the first packet) also impact non-simulcast?

[cptpcrd]

@cptpcrd - does your issue #1 (consuming the first packet) also impact non-simulcast?

As far as I'm aware, no.