only read multiple packets at a time after handshake completion

[marten-seemann]

It's more important that acknowledge packets in a timely manner before handshake completion. On the server side, we should send out the HANDSHAKE_DONE frame as soon as possible.

The GitHub diff looks like a big mess, but this is really just wrapping the receiveLoop in a if s.handshakeComplete.

[codecov]

Codecov Report

Merging #3041 (53270dd) into master (74569a5) will decrease coverage by 0.01%.
The diff coverage is 72.73%.

@@            Coverage Diff             @@
##           master    #3041      +/-   ##
==========================================
- Coverage   86.17%   86.16%   -0.01%     
==========================================
  Files         132      132              
  Lines        9354     9355       +1     
==========================================
  Hits         8060     8060              
- Misses        937      938       +1     
  Partials      357      357              

Impacted Files	Coverage Δ
session.go	77.78% <72.73%> (+0.02%)	⬆️
internal/utils/rand.go	62.50% <0.00%> (-12.50%)	⬇️

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