Invent WAL timelines, as per recent discussion, to make point-in-time

Invent WAL timelines, as per recent discussion, to make point-in-time
recovery more manageable.  Also, undo recent change to add FILE_HEADER
and WASTED_SPACE records to XLOG; instead make the XLOG page header
variable-size with extra fields in the first page of an XLOG file.
This should fix the boundary-case bugs observed by Mark Kirkwood.
initdb forced due to change of XLOG representation.