Small cleanup in db.sqlalchemy.api.action_finish()

This function was pulling back a row, updating the returned object,
then returning a reference to it. This potentially races, as the
object may be updated in another transaction after fetching. In
practise it's probably fine, but the new code is more obviously
correct, and probably slightly nicer to sqlalchemy.

Change-Id: I879e973180278faa51e664d526d75e5f095a64ea
Merge "Small cleanup in db.sqlalchemy.api.action_finish()"