Typing cleanups 2 - Make all the handlers inherit from IHandler protocol to remove casts, consolidate tests.

[ThosRTanner]

Fixes #828 (oddly)

Depends on #801

Why is this needed?

This makes the type checking safer and allows us to remove a lot of casting.

Proposed Changes

• Make all the SequentialHandlers inherit from the IHandler Protocol
• Clean up the IAsyncResult protocol and uses of it.
• ...

Does this PR introduce any breaking change?

No.

[codecov]

Codecov Report

❌ Patch coverage is 72.94118% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.44%. Comparing base (f54c4fd) to head (d719a7a).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
kazoo/handlers/gevent.py	13.51%	32 Missing ⚠️
kazoo/handlers/eventlet.py	0.00%	13 Missing ⚠️
kazoo/protocol/connection.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #829      +/-   ##
==========================================
- Coverage   94.90%   90.44%   -4.47%     
==========================================
  Files          27       27              
  Lines        3811     3799      -12     
==========================================
- Hits         3617     3436     -181     
- Misses        194      363     +169     

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