Fix salt comment, add salt and HLL tests, update README

The SaltStore comment said yesterday's salt is deleted. The code keeps
today's and yesterday's and deletes older ones. New tests check salt
rotation and reopening a file database, and the HLL switch from exact
to dense at 512/513 hashes.

README: drop the three fixed limitations, describe the live tolerance
and per-monitor timers, fix moved line links and test counts.

Author: sinnercode228