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>
(cherry picked from commit 434af9b45cfb069ab6a59c14ccaa918f96fba8a3)
excalibur.pg.py: Logic error.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
(cherry picked from commit 50bbe5f6543c1163da14805dad5153e8157c9359)
excalibur: Another logic error.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
(cherry picked from commit e2ca0986e103b78734f1da7ffab9fc6792afc818)
excalibur: Stupid programmer error.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
(cherry picked from commit 0bd2cb564835a9e011fc71279109965c84cc8f37)
excalibur.mysql: Applied ETag/Last-Modified changes. Untested.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
(cherry picked from commit 829e7f35d6d2a7950f11b6f8296d2179c997a2d7)