Tidy up the rest-proxy transport

[cddr]

I'd like to add a capability to the rest-proxy transport that allows a test-author to start consuming the specified topics from the beginning rather than the end. However before getting to that, I wanted to tidy up a few niggling issues so this PR tries to address those.....

• Replace some blocking HTTP calls with asynchronous calls that deal with the response in a callback. This seems to make the rest-proxy transport test that was failing intermittently fail more reliably so...
• Add a small (200ms) delay to the rest-proxy transport constructor which seems to prevent the failure described above from happening at all. I'd love merge a non-sleepy fix to this problem but I'm starting to wonder if it might actually caused by a race condition in the rest-proxy itself.
• Add a type hint to prevent the compile warnings caused by the recently added suppression stuff

[codecov[bot]]

Codecov Report

Merging #205 into master will increase coverage by 0.13%. The diff coverage is 82.08%.

@@            Coverage Diff             @@
##           master     #205      +/-   ##
==========================================
+ Coverage   79.13%   79.27%   +0.13%     
==========================================
  Files          41       41              
  Lines        2531     2543      +12     
  Branches      151      150       -1     
==========================================
+ Hits         2003     2016      +13     
  Misses        377      377              
+ Partials      151      150       -1

Impacted Files	Coverage Δ
src/jackdaw/streams/interop.clj	80.28% <100%> (ø)	:arrow_up:
src/jackdaw/test/transports/rest_proxy.clj	84.23% <81.81%> (+1.51%)	:arrow_up:

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