Add Generic WAL interface

Add Generic WAL interface

This interface is designed to give an access to WAL for extensions which
could implement new access method, for example. Previously it was
impossible because restoring from custom WAL would need to access system
catalog to find a redo custom function. This patch suggests generic way
to describe changes on page with standart layout.

Bump XLOG_PAGE_MAGIC because of new record type.

Author: Alexander Korotkov with a help of Petr Jelinek, Markus Nullmeier and
	minor editorization by my
Reviewers: Petr Jelinek, Alvaro Herrera, Teodor Sigaev, Jim Nasby,
	Michael Paquier