allow the first key update immediately after handshake confirmation

[marten-seemann]

Fixes #2810.

[codecov]

Codecov Report

Merging #2811 into master will increase coverage by 0.02%.
The diff coverage is 79.31%.

@@            Coverage Diff             @@
##           master    #2811      +/-   ##
==========================================
+ Coverage   85.92%   85.94%   +0.02%     
==========================================
  Files         133      133              
  Lines       12075    12074       -1     
==========================================
+ Hits        10375    10376       +1     
+ Misses       1365     1363       -2     
  Partials      335      335              

Impacted Files	Coverage Δ
internal/handshake/crypto_setup.go	64.88% <0.00%> (-0.26%)	⬇️
internal/handshake/updatable_aead.go	96.13% <85.00%> (-0.06%)	⬇️
session.go	76.97% <100.00%> (+0.33%)	⬆️

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