Expose a count of connected peers which are currently syncing data cores

[gmaclennan]

Description

Currently the sync API only exposes an integer connectedPeers, which is the number of peers that a device is connected to.

The API does not expose which of these peers are actively syncing, either the initial core, or the data cores.

It would be valuable for the UX to be able to show how many of the connected peers are currently syncing data cores. A future iteration of the UX might want to show a list of connected peers and which ones are syncing.

This issue is to add a count of connected & syncing data peers. A list of peers can be added to the API at a later stage.

Tasks

I'm not sure about how to implement this at this stage. Ideas welcome!