fix race when receiving mail during rule process

fix race when receiving mail during rule process

Get the state (uidnext) for each inbox used in the configuration.
It gives the uid of the next message the will be added in the mbox.
We will only care about messages with a uid lower than this uidnext,
to avoid race conditions with a message arriving while we are in the

Signed-off-by: Olivier Matz <zer0@droids-corp.org>