* Clarify what the first option for the marker does.
* Fix unsetting a marker deliberately
* Fix several bugs with updating marker associations after changing a term.
* Fix SQL error when setting a marker on existing terms.
* Add function to recompute markers after changing a term.

* Delete marker associations properly.
* Join to {node_revisions} properly.
* Handle upgrading from a 5.x version after the changed I just made in that branch.