unifi-check.pl: remove dead code
unifi-check.pl: be explicit in our usage of SQL::Abstract
unifi-check.pl: whitespace
unifi-check.pl: remove connection flapping timeout code
unifi-check.pl: SELECT DISTINCT, so we don't have to uniq the results
unifi-check.pl: make unifi_request 'session' unique to the host

This paves the way to connect to multiple Unifis in the future.
$UNIFI_CTRL is still global, though.
unifi-check.pl: move DB updates into update_benutzerdb_lease()
unifi-check.pl: move status API code into post_status_update()
unifi-check.pl: read secrets from a config file