excalibur.py: Every tick, scan requests older than reqexpire are marked inactive.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
excalibur.py: Found a prettier way to do request expiring.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
excalibur: Added request expiring to MySQL excalibur.

Signed-off-by: Martin Stone <martin@d7415.co.uk>