Simplify event stream

[tomasaschan]

The most disruptive change here is to revert the rename of IEventStore to IEventStream; I found it to be confusing that we have an IEventStream that can then query the store for events across multiple streams... Thus, the interface (and its implementations) are now called IEventStore and I intend to keep it that way.

It might make sense to create an abstraction at some point that is something like this:

public interface IEventStream<TEvent, TEventMetadata>
{
    Task<IEnumerable<TEvent>> Events(Func<IQueryable<TEventMetadata>, IQueryable<TEventMetadata>> query);

    Task Append(long versionBefore, IEnumerable<object> payloads);
}    

and add a method to the event store that gets you one, like

public interface IEventStore<TStreamId, TEvent, TEventMetadata>
{
    // current members...

    IEventStream<TEvent, TEventMetadata> GetStream(TStreamId streamId);
}

where the implementation is basically just a wrapper around the store and the stream identifer.