Add enable_timeout_every() to fire the same timeout repeatedly.
enable_timeout_at() and enable_timeout_after() can still be used
when you want to fire a timeout just once.
Patch by me, per a suggestion from Tom Lane.
Discussion: http://postgr.es/m/
2992585.
1632938816@sss.pgh.pa.us
Discussion: http://postgr.es/m/CA+TgmoYqSF5sCNrgTom9r3Nh=at4WmYFD=gsV-omStZ60S0ZUQ@mail.gmail.com