Add data closure to the cycle_callback of deferred response

[etr]

Description of the Change

This change adds a data closure that is passed through the cycle_callback of a deferred response.

Alternate Designs

A couple of pull requests have been proposed to tackle this:

• Add private data support for deferred_response #144
• Make cycle_callback a functor (+ rename cycle_callback -> data_callback) #73

Possible Drawbacks

This adds a template to the deferred_response effectively complicating the interface.

Verification Process

Performed tests on travis.

Release Notes

It is now possible to pass a data closure through the cycle_callback of a deferred response.

[codecov-io]

Codecov Report

Merging #145 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   96.46%   96.55%   +0.09%     
==========================================
  Files          34       34              
  Lines        2995     3019      +24     
==========================================
+ Hits         2889     2915      +26     
+ Misses        106      104       -2

Impacted Files	Coverage Δ
test/integ/deferred.cpp	100% <100%> (ø)	⬆️
src/httpserver/deferred_response.hpp	100% <100%> (ø)	⬆️
src/deferred_response.cpp	100% <100%> (+16.66%)	⬆️

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