Use events internally to simplify version-specific EZSP logic

[puddly]

These changes are broken out from #704.

As part of a gradual migration within zigpy, I'd like to start using events internally to simplify our ad-hoc callback-based and name-based event registration systems. The various EZSP versions have different internal command field ordering, struct fields in general, logic in general, etc. This is currently handled messily within the main ControllerApplication subclass.

This PR migrates most logic into the protocol version-specific handlers.

Bellows will require future work to tediously migrate to dedicated request/response objects, which will allow us to use their types more effectively and drop these intermediate events.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.56%. Comparing base (9e31738) to head (9f10b69).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #706      +/-   ##
==========================================
+ Coverage   99.53%   99.56%   +0.02%     
==========================================
  Files          58       58              
  Lines        4128     4179      +51     
==========================================
+ Hits         4109     4161      +52     
+ Misses         19       18       -1     

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