generate metadata dumps as first pass, rev text as second pass dumps
[WIP] backupPrefetch.inc will properly locate revisions, under the
contstraints that boards are in ascending order in the dump file, topics
are in ascending order in a board, posts at same tree level are in
ascending order. This is not currently true but it's required for
prefetch as a concept to work.
dumpTextPass.php is not yet rewritten to work with the new backupPrefetch.
This cannot be merged until a change to the regular dump maintenance scripts
is merged in core.