snapshot scalability: cache snapshots using a xact completion counter.

 snapshot scalability: cache snapshots using a xact completion counter.
 
 Previous commits made it faster/more scalable to compute snapshots. But not
 building a snapshot is still faster. Now that GetSnapshotData() does not
 maintain RecentGlobal* anymore, that is actually not too hard:
 
 This commit introduces xactCompletionCount, which tracks the number of
 top-level transactions with xids (i.e. which may have modified the database)
 that completed in some form since the start of the server.
 
 We can avoid rebuilding the snapshot's contents whenever the current
 xactCompletionCount is the same as it was when the snapshot was
 originally built.  Currently this check happens while holding
 ProcArrayLock. While it's likely possible to perform the check without
 acquiring ProcArrayLock, it seems better to do that separately /
 later, some careful analysis is required. Even with the lock this is a
 significant win on its own.
 
 On a smaller two socket machine this gains another ~1.03x, on a larger
 machine the effect is roughly double (earlier patch version tested
 though).  If we were able to safely avoid the lock there'd be another
 significant gain on top of that.
 
 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