MBS 5744: quicker modbot

http://tickets.musicbrainz.org/browse/MBS-5744 -- see my comments.

This patch makes it so that ModBot only looks at edits that have *any* chance of closing: i.e., either are past their expiration time (expire_time < now()) or have at least 3 unanimous votes. I also added a note to the place where the 3-votes rule is defined noting to update EditQueue if that number changes.