Suppress useless wakeups in walreceiver.

Suppress useless wakeups in walreceiver.

Instead of waking up 10 times per second to check for various timeout
conditions, keep track of when we next have periodic work to do.

Author: Thomas Munro <[email protected]>
Author: Nathan Bossart <[email protected]>
Reviewed-by: Kyotaro Horiguchi <[email protected]>
Reviewed-by: Bharath Rupireddy <[email protected]>
Reviewed-by: Alvaro Herrera <[email protected]>
Discussion: https://postgr.es/m/CA%2BhUKGJGhX4r2LPUE3Oy9BX71Eum6PBcS8L3sJpScR9oKaTVaA%40mail.gmail.com