Ensure that BEFORE STATEMENT triggers fire the right number of times.

Ensure that BEFORE STATEMENT triggers fire the right number of times.

Commit 0f79440fb introduced mechanism to keep AFTER STATEMENT triggers
from firing more than once per statement, which was formerly possible
if more than one FK enforcement action had to be applied to a given
table.  Add a similar mechanism for BEFORE STATEMENT triggers, so that
we don't have the unexpected situation of firing BEFORE STATEMENT
triggers more often than AFTER STATEMENT.

As with the previous patch, back-patch to v10.

Discussion: https://postgr.es/m/22315.1505584992@sss.pgh.pa.us