implement DPLPMTUD

[marten-seemann]

Fixes #94.

[codecov]

Codecov Report

Merging #3028 (765d26f) into master (2c45f2b) will decrease coverage by 0.12%.
The diff coverage is 79.14%.

@@            Coverage Diff             @@
##           master    #3028      +/-   ##
==========================================
- Coverage   86.16%   86.04%   -0.12%     
==========================================
  Files         132      133       +1     
  Lines        9355     9433      +78     
==========================================
+ Hits         8060     8116      +56     
- Misses        938      955      +17     
- Partials      357      362       +5     

Impacted Files	Coverage Δ
internal/congestion/cubic.go	100.00% <ø> (ø)
internal/protocol/protocol.go	100.00% <ø> (ø)
session.go	76.62% <36.00%> (-1.15%)	⬇️
internal/congestion/cubic_sender.go	86.61% <73.91%> (-0.19%)	⬇️
internal/ackhandler/sent_packet_handler.go	77.86% <75.00%> (-0.14%)	⬇️
packet_packer.go	84.43% <85.37%> (-0.22%)	⬇️
buffer_pool.go	90.48% <100.00%> (ø)
config.go	100.00% <100.00%> (ø)
conn.go	100.00% <100.00%> (ø)
conn_ecn.go	70.97% <100.00%> (ø)
... and 6 more

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