As pointed out by Heikki, maintaining another variable to match one in
the database system will be error-prone and cumbersome, especially while
merging with upstream. This commit initializes ORCA with a pointer to a
GPDB function that returns true when QueryCancelPending or
ProcDiePending is set. This way we no longer have to micro-manage
setting and re-setting some internal ORCA variable, or touch signal
handlers.

This commit also reverts commit 0dfd0ebc "Support optimization interrupts
in ORCA" and reuses tests already pushed by 916f460f and 0dfd0ebc.