Said method can out-live the factory instance. This was not a problem
because the method takes care to keep all its need from `this` alive, by
copying them to the coroutine stack. However, this fact that this method
can out-live the instance is not obvious, and an unsuspecting developer
(me) added a new member (_logger) which was not kept alive.
This can cause a use-after-free in the factory. Fix by making
initialize() static, forcing the instance to pass all parameters
explicitely and add a comment explaining that this method can out-live
the instance.
