SLRU_PAGES_PER_SEGMENT as configure parameter

Hi,
The constant SLRU_PAGES_PER_SEGMENT defines the size of the SLRU
segments. It is currently hardcoded in slru.h. It would be nice to be
able to set this parameter during configuration (for example, by
analogy with --with-segsize-blocks).
What will it give us:
1) The ability to test pg_upgrade more extensively, since without it
you need to generate too much data to fill a significant number of
segments.
2) The number of segments is arbitrary: 32. However, I have not heard
about any estimates in terms of performance; with such a patch (for
REL_17_STABLE), it will be realistic to do this.

--
Best regards,
Daniil Davydov