add an internal queue to signal that a datagram frame has been dequeued

[mengelbart]

Fixes #3075

[codecov]

Codecov Report

Merging #3081 (bcc7baf) into master (eb3e100) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #3081      +/-   ##
==========================================
- Coverage   86.04%   86.03%   -0.01%     
==========================================
  Files         132      132              
  Lines        9435     9441       +6     
==========================================
+ Hits         8118     8122       +4     
- Misses        955      957       +2     
  Partials      362      362              

Impacted Files	Coverage Δ
datagram_queue.go	88.89% <75.00%> (-4.44%)	⬇️

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[marten-seemann]

Thank you @mengelbart!