koushin now takes a list of upstream URLs instead of an IMAP and SMTP
URL. This allows to specify upstream server URLs for plugins. In the
future, this will allow for auto-discovering upstream servers based on a
single domain name.
References: https://todo.sr.ht/~sircmpwn/koushin/49
There's no way around having a global mutex, because we need to update
the HTTP routes when reloading plugins. During reload we need to lock
the whole server.
Closes: https://todo.sr.ht/~sircmpwn/koushin/43
