river.py: use three decimal places for metadata.secs

To better match the spec.
simplify how limits are handled

In particular, Rename OUTPUT_LIMIT to ENTRIES_LIMIT to better reflect
what it's limiting.
download.py: limit number of items for new feeds

This prevents newly added feeds from putting 10, 25, 50 items in the
river and drowning everything out.
river.py: display how many updatedFeed objects there are