Truncate the predicate lock SLRU to empty, instead of almost empty.

Truncate the predicate lock SLRU to empty, instead of almost empty.

Otherwise, the SLRU machinery can get confused and think that the SLRU
has wrapped around.  Along the way, regardless of whether we're
truncating all of the SLRU or just some of it, flush pages after
truncating, rather than before.

Kevin Grittner