If something changes the cache behind our back (e.g. deleting or updating the
file) we need to reload the list of remotes so that the age is correctly shown.
Note: we have to transfer the mtime (not the age) when creating the GVariant,
as we want calls to fwupd_remote_get_age() to update the value without getting
the remote from the daemon each time.
Add the concept of 'remotes' that can dropped into /etc and used as firmware
metadata sources. This may be desirable when firmware is only accessable with
a valid support contract or from behind a VPN.
