Add regression test for TOCTOU race in DNS UPDATE SSU handling

Add regression test for TOCTOU race in DNS UPDATE SSU handling

Race rndc reconfig (toggling between allow-update and update-policy)
against a stream of DNS UPDATEs for 5 seconds and verify that named
does not crash.

Before the fix, the race between send_update() and update_action()
reading the SSU table independently could trigger an assertion
failure (INSIST) when the zone's update policy changed between the
two reads.

(cherry picked from commit c503b6eee8e0b7ae9c12af1b6ab2630926f6648d)