[tlbuild] Asymptote version mismatch

[Karl Berry]

    Karl is much faster with updating than the catalogue
    updates, 

BTW, that's not usually the case any more. Nowadays CTAN updates the
Catalogue more or less simultaneously with installing new uploads.

    so the first would be the real, the second only for the
    catalogue change

But this can still happen pretty frequently, either because the cat
update was delayed for whatever reason, or because of other changes to
the Catalogue that happen independently.

    we don't bump the version number of the TL packages if only
    catalogue data changes

How often to "force" Catalogue updates remains an open question, in this
new world of versioned tarballs. So far I've been doing it more or less
randomly, every few days. Letting it go on a long time is not desirable
since that would make for a fairly huge update all at once. But too
short a time is not desirable either, for the reasons above. Basically
stuck. -k