Remove lower limit on checkpoint_timeout?

Hi,

for testing - like yesterday's 6ef2eba3f - it's annoying that
checkpoint_timeout has 30s minimum. I've now locally patched that to be
1s a significant number of times.

While it's not a particularly good idea to set it to 1s on a production
system, I don't see why we need to prevent that. It's not like 30s is
likely to be a good idea either.

Hence I'd like to set the lower limit to 1s.

Comments?

Andres