The connections service no longer depends directly on the
syncthing model object, but on an interface instead. This
makes it drastically easier to write clients that handle
the model differently, but still want to benefit from
existing and future connections changes in the core.
This was motivated by burkemw3's interest in creating a
FUSE client that can present a view of the global model,
but not have all of the file data locally.
The actual decoupling was done by adding a connections.Model
interface. This interface is effectively an extension of the
protocol.Model interface that also handles connections
alongside the modified service.
