Remaining 008pre1 items done; deferred where more design is needed.

Remaining 008pre1 items done; deferred where more design is needed.
More docs and (way more!) testing needed.

Done:
 - Authdirservers down directories from others.
 - Generate and use running-routers lists
 - Cache directories; store across reboots.
 - Refactor directory parsing a bit; note potential trouble spots.

svn:r1985