Add "date" output field to Rhizome commands

To wit: "add file" and "import bundle"
Improve test defs: extract_stdout_keyvalue_optional()

Logs its variable assignment
Fix HTTP /restful/rhizome/bundlelist.json

Write test case assertions using jq(1) utility, increase from four
bundles to 100.

Fix bugs in HTTP server content generation logic.

Make payload content generator read payload 4KiB at a time, to
always read on filesystem block boundaries for performance.  Increase
size of payload in relevant test case.