Add rack active RTT measurement

[philipch07]

Description

According to https://duepublico2.uni-due.de/servlets/MCRFileNodeServlet/duepublico_derivate_00073893/Diss_Weinrank.pdf#page=113 section 7.3.2, an enhancement for RACK in SCTP is to add active RTT measurement.

This branch adds that feature. Note that it's built on the RACK branch merged with the adaptive burst mitigation branch.

The best thing to add after this would be the I bit as mentioned in 7.3.4 which is formalized in RFC 9260. That's something that will take place after all the 9260 prs go in, as it involves updating Association.go which really needs a refactor since it's way too unwieldy...

Reference issue

A RACK enhancement!

[codecov]

Codecov Report

❌ Patch coverage is 80.09119% with 131 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.32%. Comparing base (b1a66a4) to head (8f1aa04).

Files with missing lines	Patch %	Lines
association.go	79.08%	86 Missing and 33 partials ⚠️
chunk_heartbeat_ack.go	71.42%	4 Missing and 4 partials ⚠️
chunk_heartbeat.go	85.18%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #438      +/-   ##
==========================================
+ Coverage   83.74%   84.32%   +0.58%     
==========================================
  Files          51       52       +1     
  Lines        4527     4032     -495     
==========================================
- Hits         3791     3400     -391     
+ Misses        597      450     -147     
- Partials      139      182      +43     

Flag	Coverage Δ
go	84.32% <80.09%> (+0.58%)	⬆️

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