Use pg_pread() and pg_pwrite() in slru.c.
This avoids lseek() system calls at every SLRU I/O, as was
done for relation files in commit
c24dcd0c.
Reviewed-by: Ashwin Agrawal <aagrawal@pivotal.io>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2Biqke4uTRFj8D8uEUUgj%2BRokPSp%2BCWM6YYzaaamG9Wvg%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2BhUKGJ%2BoHhnvqjn3%3DHro7xu-YDR8FPr0FL6LF35kHRX%3D_bUzg%40mail.gmail.com