only arm the application data PTO timer after the handshake is confirmed

[marten-seemann]

Fixes #2688.

[codecov]

Codecov Report

Merging #2689 into master will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #2689      +/-   ##
==========================================
- Coverage   86.74%   86.73%   -0.01%     
==========================================
  Files         124      124              
  Lines        9905     9905              
==========================================
- Hits         8592     8591       -1     
- Misses        980      981       +1     
  Partials      333      333              

Impacted Files	Coverage Δ
session.go	77.35% <0.00%> (-0.10%)	⬇️
internal/ackhandler/sent_packet_handler.go	73.10% <100.00%> (ø)

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