[swift4] Add ability to pass in a dedicated queue for processing network response (Fix for 230)

[aplikianer]

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Description of the PR

Fix for #230

Updates Swift4 templates to support the ability to pass in a dedicated queue for processing network response

cc @jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @d-date (2018/03) for review

[joeboyscout04]

@aplikianer I'm interested in getting this merged also. Could you resolve the conflicts and see if we can get a maintainer to merge?

[4brunu]

@aplikianer A similar PR #4591 as been merged that does something kind of similar.
It allows to choose a global DispatchQueue that is used to process all network responses.
Your PR also allows to choose a DispatchQueue per request.
Is #4591 enough for you, or do you still need this PR?
If you need this PR, can you please fix the conflicts?

[wing328]

The fix is available in the newly-added swift5 generator so closing this one instead.