Track request result success timestamp

[relu91]

It might be useful to get and store the latest timestamp when using trackRequestResult and joinRequestResult.

[relu91]

Meanwhile I come up with this simple solution:

const tracked$ = store.pipe(selectAllEntities(), joinRequestResult(['key'])
const lastUpdate$ = tracked$.pipe(
      pairwise(),
      filter(
        ([{ fetchStatus: previous }, { fetchStatus: current }]) =>
          previous === 'fetching' && current === 'idle',
      ),
      map(() => new Date()),
      shareReplay(1),
    );

FYI I had to add shareReplay(1) for subscribers that subscribed after the first transition from 'fetching' to 'idle' happened. There might be other edge cases but it worked for mine.