Using multiple NTP servers (#6173)

* Small improvements to time estimation.

* Allow multiple NTP servers to be used.

* Removing boxing.

* Be nice.

* Be nicer.

* Update list of servers and add reference.

dapps/src/tests/helpers/mod.rs

@@ -255,7 +255,7 @@ impl Server {
 		fetch: F,
 	) -> Result<Server, http::Error> {
 		let middleware = Middleware::dapps(
-			"pool.ntp.org:123",
+			&["0.pool.ntp.org:123".into(), "1.pool.ntp.org:123".into()],
 			CpuPool::new(4),
 			remote,
 			signer_address,