* Refactor mysensors message handling
* Add handler module and register handlers per message type or message
sub-type. This will allow easier extension of message handling in the
future.
* Move some common functions to a helpers module.
* Add node handler and signal
* Fix inconsistent return
* Upgrade pymysensors to 0.18.0
* Fix bug in message modification.
* Fix mysensors connection task blocking setup
* Schedule the connection task without having the core track the task
to avoid blocking setup.
* Cancel the connection task, if not cancelled already, when
home assistant stops.
* Use done instead of cancelled
## Description:
More fixes flagged by pylint 2 that don't hurt to have before the actual pylint 2 upgrade (which I'll submit soon).
## Checklist:
- [ ] The code change is tested and works locally.
- [x] Local tests pass with `tox`. **Your PR cannot be merged unless tests pass**
