use a Peek / Pop API for the datagram queue

[marten-seemann]

Fixes #3550.

This makes it easier to reason about the concurrency properties of the datagram queue. I've been unable to understand how the bug described in #3550 actually arises, but this seems change seems to fix it.

It will also allow us to pack multiple DATAGRAMs into the same QUIC packet, if we desire to do so.

[codecov]

Codecov Report

Base: 85.50% // Head: 85.49% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (3399ebc) compared to base (424a663).
Patch coverage: 86.36% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3582      +/-   ##
==========================================
- Coverage   85.50%   85.49%   -0.01%     
==========================================
  Files         139      139              
  Lines       10244    10237       -7     
==========================================
- Hits         8759     8752       -7     
  Misses       1103     1103              
  Partials      382      382              

Impacted Files	Coverage Δ
datagram_queue.go	85.37% <84.62%> (-6.12%)	⬇️
packet_packer.go	88.18% <87.50%> (+0.41%)	⬆️
connection.go	76.44% <100.00%> (ø)

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