Replace check result dictionaries with a class.

Refs #5039
Ignore invalid fields in the state file.

Refs #5039
Implement class for service comments.

Refs #5039
Implement class for service downtimes.

Refs #5039
Refactor the Service::Is{Comment,Downtime}* methods.

Refs #5039
Merge branch 'feature/checkresult-5039' into next

Fixes #5039