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.
The code parsing all the possible errors should go into bugzilla service. Bugzilla service knows what is the reason of an error. Separate that logic. It goes well with converting error codes to exceptions