tcp: fix retransmit exponential backoff, align to rfc6298.

[Dirbaio]

Align RTT estimation and RTO calculation to RFC 6298.

In particular, this fixes exponential backoff in retransmissions, because
the RTO is supposed to be stored in a persistent variable so we can double it on
retransmission and have it "stick", while we were storing it in the timer which would
get cleared every time we exited retransmit state.

closes #1020

• tcp: emsure line numbers in tests point to the actual failure.
• tcp: fix retransmit exponential backoff, align to rfc6298.
• tcp: add retransmission exponential backoff test.

[codecov]

Codecov Report

Attention: Patch coverage is 98.24561% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.88%. Comparing base (37a6e1c) to head (e826989).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/socket/tcp.rs	98.24%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1023      +/-   ##
==========================================
+ Coverage   80.83%   80.88%   +0.05%     
==========================================
  Files          81       81              
  Lines       28419    28447      +28     
==========================================
+ Hits        22973    23010      +37     
+ Misses       5446     5437       -9     

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