Bugzilla plugin is now converted to backend sync service

as the summary, Bugzilla is now a backend sync service. It can be used simply as it was a plugin before. When enable this service, any added task will be handled and attempt to sync bug information from specific related Bugzilla service.

This version works well, but not perfect. I'm being continue to add more features, such as the configuration ability.
Yup, reason is I want to give a more descriptive text, that would be used in log or report to user perhaps. ``KeyError: ....`` is not helpful clearly to know what happens.