Instead of storing Message-IDs in the Msgmap object, we can
store the blob OID.
For initial indexing of mirrors, this lets us preserve
$sync->{regen} by storing the intended article number in
the queue.
On --reindex, the article number we store in Msgmap is ignored
but only used for ordering purposes.
This also allows us to avoid ENOMEM errors if somebody abuses
our system by reusing Message-IDs; but we now risk ENOSPC
instead (but systems tend to have more FS storage than RAM).