update-pkgcache.py needs to connect as a unique database user

update-pkgcache.py is currently connecting as the statistician database user, which makes it impossible to tell its activity from the update-stats.py script.

All scripts should connect as a unique database user.

This involves adding the new user to security.cfg (it can inherit permissions from the statistician user for a quick fix) and hardcoding the new database user into cronscripts/update-pkgcache.py (there isn't much point configuring database usernames in config files because we have never overridden them in the last 5 years).