Add flush to persist api

[patrickhousley]

When providing a custom persistence storage that includes async calls, there is no way to tell through the store interface that the persist has completed. For example, using indexDB as the storage with custom get/set/remove functions passed in the persist options, it would be nice if we could call await store.persist.flush().

[dai-shi]

no way to tell through the store interface that the persist has completed.

Yeah, that's correct.

store.persist.flush()

Not sure if flush is the right API. Maybe pending(): Promise<void>?