Improve interaction of vectx and tftp

On slow platforms, it helps to spread the hashing load
over time so that tftp does not timeout.

Also, some .4th files are too big to fit in cache of pkgfs,
so increase cache size and ensure fully populated.

Reviewed by: stevek
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24287