Asynchronously reads the highest stored sequence number for provided persistenceId. The persistent actor will use the highest sequence number after recovery as the starting point when persisting new events. This sequence number is also used as `toSequenceNr` in subsequent calls to ReplayMessagesAsync(IActorContext, String, Int64, Int64, Int64, ActionIPersistentRepresentation) unless the user has specified a lower `toSequenceNr`. Journal must maintain the highest sequence number and never decrease it. This call is protected with a circuit-breaker. Please also not that requests for the highest sequence number may be made concurrently to writes executing for the same persistenceId, in particular it is possible that a restarting actor tries to recover before its outstanding writes have completed.

Task<long> ReadHighestSequenceNrAsync(
	string persistenceId,
	long fromSequenceNr
)

abstract ReadHighestSequenceNrAsync : 
        persistenceId : string * 
        fromSequenceNr : int64 -> Task<int64>