Remove unused elements from struct rhizome_http_request

Also improve comments
Make Content-Length optional in HTTP responses

To eventually support the /restful/rhizome/bundlelist.json query which
will not compute in advance the length of its response.
Improve Rhizome list cursor

Remove 'rowcount' element

Order by descending ROWID, which is functionally the same as descending
inserttime but more reliable

Replace '_offset' cursor element with rowid first-last range to record
the expanding window of rows already returned, which allows release and
re-open of cursor mid-listing without missing rows or producing duplicates