Fix not sending MasterKeyIdentifier in the use_srtp extension

[theodorsm]

Description

Currently, the use_srtp extension is missing the configured MasterKeyIdentifier in flight 3 (the second ClientHello after a HelloVerifyRequest)

Also updated the comment above UseSRTP to point to the correct RFC.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.46%. Comparing base (fd27a52) to head (e6957b0).

Additional details and impacted files

@@             Coverage Diff              @@
##           dtls-1.2     #835      +/-   ##
============================================
+ Coverage     82.44%   82.46%   +0.01%     
============================================
  Files           121      121              
  Lines          6928     6929       +1     
============================================
+ Hits           5712     5714       +2     
+ Misses          805      804       -1     
  Partials        411      411              

Flag	Coverage Δ
go	82.46% <100.00%> (+0.01%)	⬆️

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[JoTurk]

Thank you, Will be cool to have a regression test.

[theodorsm]

@JoTurk thanks for the quick review. We are lacking proper tests for flights currently. I want to improve that together with the new flights for dtls 1.3. I'll add this as a regressions test if I make such a PR in the future, I suspect there are more subtle bugs hiding around too.