allow an amplification factor of 3.x

[marten-seemann]

This achieves the purpose of the spec: Limit the amplification factor, and simplifies the code by always allowing full-size packets.

[codecov]

Codecov Report

Merging #2862 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2862      +/-   ##
==========================================
- Coverage   85.67%   85.67%   -0.01%     
==========================================
  Files         134      134              
  Lines        9087     9083       -4     
==========================================
- Hits         7785     7781       -4     
  Misses        960      960              
  Partials      342      342              

Impacted Files	Coverage Δ
internal/ackhandler/sent_packet_handler.go	73.05% <100.00%> (-0.13%)	⬇️
packet_packer.go	83.41% <100.00%> (-0.08%)	⬇️
session.go	77.24% <100.00%> (ø)

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