Add an RPC request for the state of the network (#1884)

* Add an RPC request for the state of the network

* Fix concerns

* Fix tests

* Replace comment with TODO

* Rename the RPC

substrate/core/network/src/lib.rs

@@ -43,6 +43,7 @@ pub use protocol::{ProtocolStatus, PeerInfo, Context};
 pub use sync::{Status as SyncStatus, SyncState};
 pub use network_libp2p::{
     NodeIndex, ProtocolId, Severity, Protocol, Multiaddr,
+	NetworkState, NetworkStatePeer, NetworkStateNotConnectedPeer, NetworkStatePeerEndpoint,
     obtain_private_key, build_multiaddr, PeerId, PublicKey
 };
 pub use message::{generic as generic_message, RequestId, Status as StatusMessage, ConsensusEngineId};