Change long_running_queries to long_running_xacts

This action has a superset of the functionality of the previous
long_running_queries action.

Long running transactions are undesirable because they make transactions hold
onto their heavyweight locks, and they prevent vacuum from reclaiming dead
rows.

It makes more sense to structure the action such that long running transactions
over 5 minutes are highlighted, since any queries taking more than 5 minutes
would naturally also occur within a transaction taking at least as long.