I've chosen the "AuthDirMaxServersPerAddr" option here for
simplicity, since it is used literally nowhere else besides the dirauth
module. Once we have all the infrastructure in place for this, we
can move more options into this structure.
When we tell the periodic event manager about an event, we are
"registering" that event. The event sits around without being
usable, however, until we "connect" the event to libevent. In the
end, we "disconnect" the event and remove its libevent parts.
Previously, we called these operations "add", "setup", and
"destroy", which led to confusion.
