Previously the exit handling was only able to exit from within the
main loop, and not from within the backend code it calls.  Fix that by
using the standard die() SIGTERM handler, and adding the necessary
CHECK_FOR_INTERRUPTS() call.

This requires adding yet another process-type-specific branch to
ProcessInterrupts(), which hints that we probably should generalize
that handling.  But that's work for another day.

Author: Petr Jelinek
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/fe072153-babd-3b5d-8052-73527a6eb657@2ndquadrant.com