Fix data race in RTX

[JoeTurki]

Description

@jech reported a data race in read in here #3328

while it doesn't happen. a comment from @arjunshajitech in the discord made me notice a data race in ReadRTX. this is a test to prove it before I add a fix.

Taking about the latest issue opened by jech. #3328
I think that with simulcast enabled, there's a high possibility of it becoming racy.
After the lower layers start reading, the higher layers come in and mutate it in receiveForRid and
receiveForRtxInternal.
Correct me if i am wrong
@arjunshajitech

Edit: this includes the fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.95%. Comparing base (d716c9b) to head (bdb7501).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3332      +/-   ##
==========================================
- Coverage   85.03%   84.95%   -0.09%     
==========================================
  Files          80       80              
  Lines        9491     9497       +6     
==========================================
- Hits         8071     8068       -3     
- Misses        999     1005       +6     
- Partials      421      424       +3     

Flag	Coverage Δ
go	84.95% <100.00%> (-0.09%)	⬇️

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