snapshot scalability: Move PGXACT->vacuumFlags to ProcGlobal->vacuumFlags.

snapshot scalability: Move PGXACT->vacuumFlags to ProcGlobal->vacuumFlags.

Similar to the previous commit this increases the chance that data
frequently needed by GetSnapshotData() stays in l2 cache. As we now
take care to not unnecessarily write to ProcGlobal->vacuumFlags, there
should be very few modifications to the ProcGlobal->vacuumFlags array.

Author: Andres Freund 
Reviewed-By: Robert Haas 
Reviewed-By: Thomas Munro 
Reviewed-By: David Rowley 
Discussion: https://postgr.es/m/20200301083601[email protected]