Add Epoch<T> and use it in for the memberships$ behavior and its

derivatives.

src/state/Behavior.ts

@@ -18,6 +18,10 @@ import { BehaviorSubject } from "rxjs";
  */
 export type Behavior<T> = Omit<BehaviorSubject<T>, "next" | "observers">;
 
+export type BehaviorWithEpoch<T> = Behavior<T> & {
+  pipeEpoch(): Behavior<{ value: T; epoch: number }>;
+};
+
 /**
  * Creates a Behavior which never changes in value.
  */