only apply server's transport parameters after handshake completion

[marten-seemann]

On the client side, 0-RTT packets have to be sent using the old transport parameters. We're only allowed to use the updated values for 1-RTT packets.

See https://tools.ietf.org/html/draft-ietf-quic-transport-34#section-7.4.1.

[codecov]

Codecov Report

Merging #3085 (d970a10) into master (9dcb56b) will increase coverage by 0.10%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master    #3085      +/-   ##
==========================================
+ Coverage   86.03%   86.13%   +0.10%     
==========================================
  Files         132      132              
  Lines        9441     9446       +5     
==========================================
+ Hits         8122     8136      +14     
+ Misses        957      949       -8     
+ Partials      362      361       -1     

Impacted Files	Coverage Δ
session.go	77.61% <89.47%> (+1.01%)	⬆️

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