Reentrant, cluster-wide keyed mutex — the non-Postgres half of Gamend.Lock.serialize/3.

:global shares a lock between holders with the same requester, so the resource goes in the resource slot and self() in the requester slot. Putting the protected id in the requester slot lets two concurrent requests for it both acquire — the exact case the lock exists to prevent.

self() also makes it reentrant, which core needs: the matchmaking sweep takes a lock and then creates a match, which takes one again. :global releases on process death.