This was XEP-0237, but is now part of RFC 6121.
Roster backends should now expose two additional methods:
version(jid):
Return the version of the given JID's roster.
set_version(jid, version):
Update the version of the given JID's roster.
A new state field will be passed to the backend if an item
has been marked for removal. This is 'removed' which will
be set to True.
