only start PMTUD after handshake confirmation

[marten-seemann]

Replacement for #3121.

Our PMTUD packets are never coalesced packets. Since we try packing coalesced packets until the handshake is confirmed, it makes sense to start PMTUD then.

[codecov]

Codecov Report

Merging #3138 (91629cd) into master (3376a9f) will decrease coverage by 0.01%.
The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master    #3138      +/-   ##
==========================================
- Coverage   85.48%   85.47%   -0.01%     
==========================================
  Files         131      131              
  Lines        9650     9650              
==========================================
- Hits         8249     8248       -1     
- Misses       1033     1034       +1     
  Partials      368      368              

Impacted Files	Coverage Δ
session.go	77.10% <77.27%> (ø)
internal/utils/rand.go	62.50% <0.00%> (-12.50%)	⬇️

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