events: make EventPriority.* usable as decorators.

this permits function definitions that do not have control over their
register() calls (e.g. Plugin.server_starting) to use a priority
other than MEDIUM.
plugins/backup: improve behaviour in strange cases.

(e.g. `~backup` immediately followed by `~stop`)
user_client: pep8 cleanup
manager: fix inconsistency between initial load and ~rehash