refactor: split clients and agent off settings

[pjenvey]

issue #632

[bbangert]

Keep the keyword declaration for clarity?

[codecov-io]

Codecov Report

Merging #945 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #945   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          57     57           
  Lines        9569   9588   +19     
=====================================
+ Hits         9569   9588   +19

Impacted Files	Coverage Δ
autopush/tests/test_router.py	100% <100%> (ø)	⬆️
autopush/tests/test_websocket.py	100% <100%> (ø)	⬆️
autopush/tests/test_integration.py	100% <100%> (ø)	⬆️
autopush/web/health.py	100% <100%> (ø)	⬆️
autopush/http.py	100% <100%> (ø)	⬆️
autopush/settings.py	100% <100%> (ø)	⬆️
autopush/main.py	100% <100%> (ø)	⬆️
autopush/router/simple.py	100% <100%> (ø)	⬆️
autopush/websocket.py	100% <100%> (ø)	⬆️
autopush/router/__init__.py	100% <100%> (ø)	⬆️
... and 2 more

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