Snapshot sync (#2047)

* PV64 sync

* Tests

* Client DB restore

* Snapshot restoration over IPC

* Upating test

* Minor tweaks

* Upating test

util/network/src/host.rs

@@ -282,6 +282,12 @@ impl<'s> NetworkContext<'s> {
 		}
 		"unknown".to_owned()
 	}
+
+	/// Returns max version for a given protocol.
+	pub fn protocol_version(&self, peer: PeerId, protocol: &str) -> Option<u8> {
+		let session = self.resolve_session(peer);
+		session.and_then(|s| s.lock().capability_version(protocol))
+	}
 }
 
 /// Shared host information