Send last PEP items to remote subscribers

When a remote subscriber becomes available, send him the last published
PEP items, as we do for local subscribers.

However, the current implementation depends on a running ejabberd_c2s
process of the publisher to send items to remote subscribers.  So, for
those, the behavior is always like it is for local subscribers when
"ignore_pep_from_offline" is set to "true".