Reintroduce network starter

[dmitry-markin]

This reverts #6400 as we are not ready yet to remove the network starter. Network starting faster than other tasks leads to race conditions, namely #6573 (comment).

The comment about why we need network starter is updated to reflect the current state of affairs.

[bkchr]

Can we not solve this issue differently? I would propose we introduce a NetworkState (or better name) variant to SyncEvent. This returns all the connected peers and is pushed to the event stream as the first event. This way grandpa et all will not miss any connected peers.

[dmitry-markin]

Yes, I like the idea of proactively pushing the state as the first message. This should do the trick. Thanks for the suggestion!