Add infrastructure for efficient LSN waiting

 Add infrastructure for efficient LSN waiting
 
 Implement a new facility that allows processes to wait for WAL to reach
 specific LSNs, both on primary (waiting for flush) and standby (waiting
 for replay) servers.
 
 The implementation uses shared memory with per-backend information
 organized into pairing heaps, allowing O(1) access to the minimum
 waited LSN. This enables fast-path checks: after replaying or flushing
 WAL, the startup process or WAL writer can quickly determine if any
 waiters need to be awakened.
 
 Key components:
 - New xlogwait.c/h module with WaitForLSNReplay() and WaitForLSNFlush()
 - Separate pairing heaps for replay and flush waiters
 - WaitLSN lightweight lock for coordinating shared state
 - Wait events WAIT_FOR_WAL_REPLAY and WAIT_FOR_WAL_FLUSH for monitoring
 
 This infrastructure can be used by features that need to wait for WAL
 operations to complete.
 
 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 <i.kartyshov@postgrespro.ru>
 Author: Alexander Korotkov <aekorotkov@gmail.com>
 Author: Xuneng Zhou <xunengzhou@gmail.com>
 Reviewed-by: Michael Paquier <michael@paquier.xyz>
 Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
 Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
 Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
 Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
 Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
 Reviewed-by: Euler Taveira <euler@eulerto.com>
 Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
 Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
 Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>