wire: write configured value of max_udp_payload_size transport parameter

[marten-seemann]

There's no reason the wire package should use the constant: It should be able to marshal any value configured.
Instead, we set the MaxUDPPayloadSize when initialising the connection.

No functional change expected.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.16%. Comparing base (66f968b) to head (7838481).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4501      +/-   ##
==========================================
+ Coverage   85.15%   85.16%   +0.01%     
==========================================
  Files         154      154              
  Lines       14768    14770       +2     
==========================================
+ Hits        12575    12578       +3     
+ Misses       1688     1687       -1     
  Partials      505      505              

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