feat: move cache to optional constructor argument, so it can be externalized

Signed-off-by: waterplea <alexander@inkin.ru>

client/lib/patch-db.ts

@@ -24,12 +24,13 @@ export class PatchDB<T extends { [key: string]: any }> {
     }
   } = {}
 
+  constructor(
+    private readonly source$: Observable<Update<T>[]>,
     readonly cache$ = new BehaviorSubject<Dump<T>>({
       id: 0,
       value: {} as T,
-  })
-
-  constructor(private readonly source$: Observable<Update<T>[]>) {}
+    }),
+  ) {}
 
   start() {
     if (this.sub) return