expirationd doesn't process tasks with zero length box.cfg.replication

[artemreyt]

Documentation says "by default expirationd should process tasks only on the writeable instance, it means that expirationd will not start task processing on a replica. Here the word 'replica' means an instance with at least one configured upstream".

But if to create an instance like:

box.cfg{
    replication = {};
    read_only = false,
}

Expirationd doesn't process tasks. But it has no configured upstream in real.

If it's really a bug, it seems to be because of this line (box.cfg.replication actually not nil):

expirationd/expirationd.lua

Line 151 in 176e713

if (box.cfg.replication_source == nil and box.cfg.replication == nil) or task.force then

Of course option force of expirationd.start() can help at this case, but In my opinion the word force you should use only where it is really needed :)

[ArtDu]

also related to #92