Add support for Reactors and Reducers to be notified when a replay begins and ends

[einari]

By having an interface like INotifyReplay:

public interface INotifyReplay
{
     Task BeginReplay();
     Task EndReplay();
}

For individual partitions we would have a separate interface:

public interface INotifyReplayPartition
{
     Task BeginReplayPartition(Key key);
     Task EndReplayPartition(Key key);
}