Re: (ITS#8209) Broken MDB_CP_COMPACT threading

[h.b.furuseth@usit.uio.no]

More:

MDB_CP_COMPACT would drop a new, empty main DB.
(I.e. the main DB would lose its flags.)

Compacting can verify that the written metapage claims
enough DB pages.  But not the opposite - it should not
fail if it claims too many pages since that would mean
we can't compact a DB which has suffered page leaks.

We could detect that and re-write the metas at the end,
if the user requests it and the file is seekable.