snapshot scalability: Move PGXACT->xmin back to PGPROC.

 snapshot scalability: Move PGXACT->xmin back to PGPROC.
 
 Now that xmin isn't needed for GetSnapshotData() anymore, it leads to
 unnecessary cacheline ping-pong to have it in PGXACT, as it is updated
 considerably more frequently than the other PGXACT members.
 
 After the changes in dc7420c2c92, this is a very straight-forward change.
 
 For highly concurrent, snapshot acquisition heavy, workloads this change alone
 can significantly increase scalability. E.g. plain pgbench on a smaller 2
 socket machine gains 1.07x for read-only pgbench, 1.22x for read-only pgbench
 when submitting queries in batches of 100, and 2.85x for batches of 100
 'SELECT';.  The latter numbers are obviously not to be expected in the
 real-world, but micro-benchmark the snapshot computation
 scalability (previously spending ~80% of the time in GetSnapshotData()).
 
 Author: Andres Freund <andres@anarazel.de>
 Reviewed-By: Robert Haas <robertmhaas@gmail.com>
 Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
 Reviewed-By: David Rowley <dgrowleyml@gmail.com>
 Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de