initialize the congestion controller with the actual max datagram size

[marten-seemann]

Fixes #3106.

[codecov]

Codecov Report

Merging #3107 (7feda78) into master (81d16a9) will decrease coverage by 0.06%.
The diff coverage is 31.25%.

@@            Coverage Diff             @@
##           master    #3107      +/-   ##
==========================================
- Coverage   85.47%   85.41%   -0.06%     
==========================================
  Files         131      131              
  Lines        9650     9663      +13     
==========================================
+ Hits         8248     8253       +5     
- Misses       1034     1042       +8     
  Partials      368      368              

Impacted Files	Coverage Δ
internal/ackhandler/ackhandler.go	0.00% <0.00%> (ø)
internal/congestion/cubic_sender.go	80.33% <16.67%> (-6.28%)	⬇️
internal/ackhandler/sent_packet_handler.go	77.75% <100.00%> (+0.06%)	⬆️
session.go	77.15% <100.00%> (+0.04%)	⬆️
internal/utils/rand.go	75.00% <0.00%> (+12.50%)	⬆️

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