Use the pairing heap instead of a flat array for LSN replay waiters

Use the pairing heap instead of a flat array for LSN replay waiters

06c418e163 introduced pg_wal_replay_wait() procedure allowing to wait for
the particular LSN to be replayed on standby.  The waiters were stored in
the flat array.  Even though scanning small arrays is fast, that might be a
problem at scale (a lot of waiting processes).

This commit replaces the flat shared memory array with the pairing heap,
which holds the waiter with the least LSN at the top.  This gives us O(log N)
complexity for both inserting and removing waiters.

Reported-by: Alvaro Herrera
Discussion: https://postgr.es/m/202404030658.hhj3vfxeyhft%40alvherre.pgsql