Clustering: DataChangeListeners may not get notified of initial data on registration

DataChangeListeners are only notified if they're on the leader node. The proxy DCL actors don't dispatch notifications to the client DCL until it gets the EnableNotifications message from the shard when it becomes the leader.

After a shard recovers from persistence, it's initialized to a follower and some time can pass before it figures out if it can become the leader. In the meantime, if a client DCL registers, it won't get notified of initial data.

This issue can easily be seen with the toaster.

Potential solution:

On DCL registration, in the Shard, if it isn't the leader, create the DCL actor but don't register with the IMDS until the shard becomes the leader.