docs: Consolidate into new "WAL for Extensions" chapter.
Previously, we had consecutive, very short chapters called "Generic
WAL" and "Custom WAL Resource Managers," explaining different approaches
to the same problem. Merge them into a single chapter. Explain most
of the differences between the approaches in the chapter's
introductory text, rather than in the individual sections.
Discussion: http://postgr.es/m/
46ac50c1-6b2a-404f-a683-
b67af6ab56e9@eisentraut.org