pg_prewarm really needs some CHECK_FOR_INTERRUPTS

Hi,

pg_prewarm() currently can't be cannot be interrupted - which seems odd
given that it's intended to read large amounts of data from disk. A
rather slow process.

Unless somebody protests I'm going to add a check to the top of each of
the three loops.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services