pgsql: Add enable_timeout_every() to fire the same timeout repeatedly.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/732e6677a667c03b1551a855e3216644b0f125ec

Modified Files
--------------
src/backend/utils/misc/timeout.c | 65 ++++++++++++++++++++++++++++++++++++----
src/include/utils/timeout.h      |  7 +++--
2 files changed, 65 insertions(+), 7 deletions(-)