excalibur.pg.py: Use If-None-Match and If-Modified-Since to reduce pa server load.
Core/maps: New Updates columns to store ETag/Last-Modified data.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
excalibur.pg.py: Logic error.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
excalibur: Another logic error.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
excalibur: Stupid programmer error.

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