Only make buffer strategy for vacuum when it's likely needed

Only make buffer strategy for vacuum when it's likely needed

VACUUM FULL and VACUUM ONLY_DATABASE_STATS will not use the vacuum
strategy ring created in vacuum(), so don't waste effort making it in
those cases.

There are other conceivable cases where the buffer strategy also won't be
used, but those are probably less common and not worth troubling over too
much.  For example VACUUM (PROCESS_MAIN false, PROCESS_TOAST false).
There are other cases too, but many of these are only discovered once
inside vacuum_rel().

Author: Melanie Plageman
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/CAAKRu_ZLRuzkM3zKogiZAz2hUony37yLY4aaLb8fPf8fgqs5Og@mail.gmail.com