Use ProcNumbers instead of direct Latch pointers to address other procs
This is in preparation for replacing Latches with a new abstraction.
That's still work in progress, but this seems a little tidier anyway,
so let's get this refactoring out of the way already.
Discussion: https://www.postgresql.org/message-id/
391abe21-413e-4d91-a650-
b663af49500c%40iki.fi