Support channels to be loaded by the webapp that manages them

Currently, channels are loaded by a single rest proxy. However, for multi webapp support, only those channels that can be managed by a certain webapp should be loaded by that webapp.

Also logic like 'isSiteUp' needs to be rewritten, as it can be that for example 2 out of 3 hst webapps are up. In that case the channel mngr should just work for the 2 sites that are live.

In case one webapp (eg /foo) is down, we need some global 'manager' that takes care of informing other users that login to not bother checking webapp /foo...until some time has passed and a recheck should be done (same holds for the live sites)