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.
@NimitS1 Sorry, do not know revamping service framework. would be better to talk about this in a specific issue. :)

to merge this and then refactor to meet the changes to service framework is fine for me.