Add pairingheap_initialize() for shared memory usage

Add pairingheap_initialize() for shared memory usage

The existing pairingheap_allocate() uses palloc(), which allocates
from process-local memory. For shared memory use cases, the pairingheap
structure must be allocated via ShmemAlloc() or embedded in a shared
memory struct. Add pairingheap_initialize() to initialize an already-
allocated pairingheap structure in-place, enabling shared memory usage.

Discussion: https://www.postgresql.org/message-id/flat/CAPpHfdsjtZLVzxjGT8rJHCYbM0D5dwkO+BBjcirozJ6nYbOW8Q@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/flat/CABPTF7UNft368x-RgOXkfj475OwEbp%2BVVO-wEXz7StgjD_%3D6sw%40mail.gmail.com
Author: Kartyshov Ivan <[email protected]>
Author: Alexander Korotkov <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Reviewed-by: Dilip Kumar <[email protected]>
Reviewed-by: Amit Kapila <[email protected]>
Reviewed-by: Alexander Lakhin <[email protected]>
Reviewed-by: Bharath Rupireddy <[email protected]>
Reviewed-by: Euler Taveira <[email protected]>
Reviewed-by: Heikki Linnakangas <[email protected]>
Reviewed-by: Kyotaro Horiguchi <[email protected]>
Reviewed-by: Xuneng Zhou <[email protected]>