Change Store.save() to return a Promise

src/store/index.ts

@@ -175,7 +175,7 @@ export interface IStore {
     /**
      * Save does nothing as there is no backing data store.
      */
-    save(force?: boolean): void;
+    save(force?: boolean): Promise<void>;
 
     /**
      * Startup does nothing.

src/store/memory.ts

@@ -329,7 +329,9 @@ export class MemoryStore implements IStore {
      * @param force - True to force a save (but the memory
      *     store still can't save anything)
      */
-    public save(force: boolean): void {}
+    public save(force: boolean): Promise<void> {
+        return Promise.resolve();
+    }
 
     /**
      * Startup does nothing as this store doesn't require starting up.

src/store/stub.ts

@@ -189,7 +189,9 @@ export class StubStore implements IStore {
     /**
      * Save does nothing as there is no backing data store.
      */
-    public save(): void {}
+    public save(): Promise<void> {
+        return Promise.resolve();
+    }
 
     /**
      * Startup does nothing.

src/sync.ts

@@ -953,7 +953,7 @@ export class SyncApi {
                 }
 
                 // tell databases that everything is now in a consistent state and can be saved.
-                this.client.store.save();
+                await this.client.store.save();
             }
         }