Rhizome needs to support channel labels to segregate data

[gardners]

Currently bundles will end up everywhere.
We need to be able to supply a priority-ordered list of "channels" that a Serval node will store.
Rhizome direct needs to transfer in that priority order.
Rhizome needs to store/discard bundles taking the priority order into account.
Configuration option needs to exist to allow setting the channel labels we care about, in priority order.
Configuration option needs to exist to allow specifying the channel labels assigned to bundles when they are created. Must be able to be overriden by supplying a pre-formed channels= line in manifest.