Change implementation of mod_offline use_mam_for_storage

Previous version was trying to determine range of messages that should
be fetched from mam by storing time when last user resource disconnected.

But that had couple edge cases that could cause problems, for example in
case of node crash we could not store information about user disconnect
and with that we didn't have data to initiate mam query.

New version don't track user disconnects, but simply ensure that we have
timestamp of first message that is gonna be put in storage, after some
measurements cost of that check with caching on top is not that costly,
and as much more robust i decided to introduce that change.