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.
These 5 are not exceptions. Just a constant to define the error code type, by following the implementation in BackendSignals.ERRNO_*. These 5 are used in set_error_code method.