pgsql: Move SLRU_PAGES_PER_SEGMENT to pg_config_manual.h

Move SLRU_PAGES_PER_SEGMENT to pg_config_manual.h

It seems plausible that someone might want to experiment with
different values. The pressing reason though is that I'm reviewing a
patch that requires pg_upgrade to manipulate SLRU files. That patch
needs to access SLRU_PAGES_PER_SEGMENT from pg_upgrade code, and
slru.h, where SLRU_PAGES_PER_SEGMENT is currently defined, cannot be
included from frontend code. Moving it to pg_config_manual.h makes it
accessible.

Now that it's a little more likely that someone might change
SLRU_PAGES_PER_SEGMENT, add a cluster compatibility check for it.

Bump catalog version because of the new field in the control file.

Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Discussion: https://www.postgresql.org/message-id/c7a4ea90-9f7b-4953-81be-b3fcb47db057@iki.fi

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3e0ae46d907dd5f36342dd288841f4502bd571f6

Modified Files
--------------
src/backend/access/transam/xlog.c | 11 +++++++++++
src/include/access/slru.h | 15 ---------------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_control.h | 2 ++
src/include/pg_config_manual.h | 10 ++++++++++
5 files changed, 24 insertions(+), 16 deletions(-)