Added the site setting long_polling_base_url , this allows you
to farm long polling to a different server.
This setting is very important if a CDN is serving dynamic content.
FIX: only poll background message bus pages once a minute
FEATURE: background_polling_interval
FEATURE: long polling pushed back to 25 seconds
FEATURE: force a poll when a bg page returns to foreground
also improve copy of message bus settings a bit
