Use Cache API to cache binary sequence data.

This speeds up loading when the user moves away from and then
revisits the map tab.

NOTE: As of this commit, there is no way to refresh or invalidate
the cache.

lib/www/client/source/src/views/Map.vue

@@ -486,7 +486,7 @@ export default {
               }
             };
 
-            self.api([url, init, cb]);
+            self.api([url, init, cb, {cache: true}]);
           });
 
         }