OF-818 Message routing to bare JID can route to negative priority resources

This commit fixes the regression, that was introduced with the addition of Message Carbons.
Message Carbons are delivered to all non-negative sessions.
If there's no Message Carbons-enabled session, it will be delivered to the highest priority session only.
Merge pull request #40 from sco0ter/of818

OF-818 Message routing to bare JID can route to negative priority resour...