Preparation for sending non-state notifications

[yhabteab]

This PR introduces/extracts some of the codes of the existing components in preparation for #188. All these changes were part of that PR, but as it was slowly getting bigger and bigger, I decided to split up those commits which can be reviewed and merged independently.

• 503b874 Contains changes for the channel plugin, that should allow us to send non-state notifications without an incident in the future and it adds also some missing doc blocks.
• 592ac09 Just adds a simple one-line debug log in which superfluous state changes are ignored.
• f9a9bec Is the somewhat bigger commit and also far more relevant than the others, as it extracts and/or decouples the entire rule and escalation filter evaluation from the incident package into a new common type and adds even more flexibility for Support sending non-state notifications without an incident #188. It makes it possible, for instance, to create test cases just for the filter evaluation part and also removes a number of obstacles for Support sending non-state notifications without an incident #188.
• 39257c4 Renames the Incident#Id field to ID as we no longer need to implement an ID() receiver on Incident type, after the contracts.Incident interface has been removed by 503b874.