Add support for service callbacks

[bdraco]

For some services we want to send all the char value
changes at once. This resolves an issue where we send
ON and then BRIGHTNESS and the light would go to 100%
and then dim to the brightness because each callback
would only send one char at a time.

For additional details about why this is needed:
home-assistant/core#32348
home-assistant/core#13695
home-assistant/core#13768
home-assistant/core#32278

The end result is that home assistant can now get sets for a service in one call and do the right thing

2020-03-19 02:14:28 DEBUG (Thread-48) [custom_components.homekit.type_lights] _set_chars: {'Brightness': 45}
2020-03-19 02:14:29 DEBUG (Thread-48) [custom_components.homekit.type_lights] _set_chars: {'On': 1, 'Brightness': 25}
2020-03-19 02:14:30 DEBUG (Thread-48) [custom_components.homekit.type_lights] _set_chars: {'On': 1, 'Brightness': 66}

[codecov-io]

Codecov Report

Merging #229 into dev will increase coverage by 0.67%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##              dev     #229      +/-   ##
==========================================
+ Coverage   62.08%   62.76%   +0.67%     
==========================================
  Files          16       16              
  Lines        1704     1716      +12     
  Branches      176      178       +2     
==========================================
+ Hits         1058     1077      +19     
+ Misses        606      594      -12     
- Partials       40       45       +5     

Impacted Files	Coverage Δ
pyhap/accessory_driver.py	65.05% <88.88%> (+2.90%)	⬆️
pyhap/characteristic.py	98.23% <100.00%> (+0.01%)	⬆️
pyhap/service.py	98.11% <100.00%> (+0.07%)	⬆️
pyhap/accessory.py	54.72% <0.00%> (+1.35%)	⬆️

[bdraco]

@ikalchev Hope you are staying safe. Any chance you could look at this one this weekend?

[ikalchev]

Overall it makes sense. We need to make sure to properly document the fact that you can register callbacks in both the service and the char, which can cause some confusion.

[bdraco]

Good call. Will send a PR to update the docs.