Push notifications refactoring

[fenek]

Patched version of #1773

[codecov]

Codecov Report

Merging #1796 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1796      +/-   ##
==========================================
- Coverage   74.48%   74.43%   -0.06%     
==========================================
  Files         290      290              
  Lines       26942    26947       +5     
==========================================
- Hits        20068    20058      -10     
- Misses       6874     6889      +15

Impacted Files	Coverage Δ
src/event_pusher/mod_event_pusher_push.erl	88.88% <100%> (-1.7%)	⬇️
src/event_pusher/mod_event_pusher_http.erl	97.14% <100%> (+0.08%)	⬆️
...t_pusher/mod_event_pusher_push_plugin_defaults.erl	100% <100%> (ø)	⬆️
src/event_pusher/mod_event_pusher.erl	90.47% <100%> (ø)	⬆️
src/event_pusher/mod_event_pusher_sns.erl	85.52% <100%> (+0.39%)	⬆️
src/event_pusher/mod_event_pusher_push_plugin.erl	100% <100%> (ø)	⬆️
..._distrib/mod_global_distrib_outgoing_conns_sup.erl	73.91% <0%> (-8.7%)	⬇️
...rc/global_distrib/mod_global_distrib_transport.erl	47.05% <0%> (-5.89%)	⬇️
src/mod_push_service_mongoosepush.erl	82.85% <0%> (-5.72%)	⬇️
src/mod_mam_odbc_async_pool_writer.erl	66.89% <0%> (-2.71%)	⬇️
... and 4 more

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