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/api/api.rs

@@ -21,7 +21,7 @@ use hyper::method::Method;
 use hyper::status::StatusCode;
 
 use api::{response, types};
-use api::time::TimeChecker;
+use api::time::{TimeChecker, MAX_DRIFT};
 use apps::fetcher::Fetcher;
 use handlers::{self, extract_url};
 use endpoint::{Endpoint, Handler, EndpointPath};
@@ -122,7 +122,6 @@ impl RestApiRouter {
 
 			// Check time
 			let time = {
-				const MAX_DRIFT: i64 = 500;
 				let (status, message, details) = match time {
 					Ok(Ok(diff)) if diff < MAX_DRIFT && diff > -MAX_DRIFT => {
 						(HealthStatus::Ok, "".into(), diff)