Use startup_time from system.connected (#300)

* Handle startup time

* Remove dead code

* Bump protocol version + FE changes

frontend/src/common/feed.ts

@@ -80,7 +80,7 @@ export namespace Variants {
       NodeHardware,
       BlockDetails,
       Maybe<NodeLocation>,
-      Timestamp
+      Maybe<Timestamp>
     ];
   }
 

frontend/src/common/index.ts

@@ -9,4 +9,4 @@ import * as FeedMessage from './feed';
 export { Types, FeedMessage };
 
 // Increment this if breaking changes were made to types in `feed.ts`
-export const VERSION: Types.FeedVersion = 30 as Types.FeedVersion;
+export const VERSION: Types.FeedVersion = 31 as Types.FeedVersion;