Introduce `chans.name[i].connected_peers` property key

[dennisklein]

This will need a bit of internal refactoring (consolidation of zmq event consumption with rate logger thread, as well as introduction of read-only properties), but will add thread-safe and async APIs on top of Channel::GetNumberOfConnectedPeers() - also needed to see this info from a plugin (e.g. currently used by built-in interactive controller via temporary PluginServices::GetNumberOfConnectedPeers).