Move fake LSN infrastructure out of GiST.

Move fake LSN infrastructure out of GiST.

Move utility functions used by GiST to generate fake LSNs into xlog.c
and xloginsert.c, so that other index AMs can also generate fake LSNs.

Preparation for an upcoming commit that will add support for fake LSNs
to nbtree, allowing its dropPin optimization to be used during scans of
unlogged relations.  That commit is itself preparation for another
upcoming commit that will add a new amgetbatch/btgetbatch interface to
enable I/O prefetching.

Bump XLOG_PAGE_MAGIC due to XLOG_GIST_ASSIGN_LSN becoming
XLOG_ASSIGN_LSN.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Reviewed-By: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/CAH2-WzkehuhxyuA8quc7rRN3EtNXpiKsjPfO8mhb+0Dr2K0Dtg@mail.gmail.com