[Imports] Limiting changesets to 10k changes.

[Kevin Kenny]

As long as JOSM can figure it out, I don't really care. I'm sure that
I've imported things that were bigger than that, but only slightly,
and it was a nail-biting experience wondering whether stuff would time
out and what I'd do if it did. If I'd understood how to get JOSM to do
a monster like http://www.openstreetmap.org/changeset/40333602 in
pieces, I'd gladly have done so. At the time, I guessed that it was
better to exceed the recommended size than to leave unclosed
multipolygon ways lying around. I might do it differently if I had it
to do over again.