Use onComplete callback instead of EventLoopFuture

[adam-fowler]

No description provided.

[codecov]

Codecov Report

Merging #59 (6fd1ba9) into main (88c127b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage   83.17%   83.17%           
=======================================
  Files          47       47           
  Lines        2371     2372    +1     
=======================================
+ Hits         1972     1973    +1     
  Misses        399      399           

Impacted Files	Coverage Δ
Sources/Hummingbird/Configuration.swift	100.00% <100.00%> (ø)
...Hummingbird/Server/Application+HTTPResponder.swift	85.29% <100.00%> (+0.44%)	⬆️

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