We try to keep the Tahoe memory footprint low by continuously monitoring the
memory consumed by common operations like upload and download.

For each currently active upload or download, we never handle more than a
single segment of data at a time. This serves to keep the data-driven
footprint down to something like 4MB or 5MB per active upload/download.

Some other notes on memory footprint:

 * importing sqlite (for the share-lease database) raised the static
   footprint by 6MB, going from 24.3MB to 31.5MB (as evidenced by the munin
   graph from 8/29/07 to 9/2/07).