Make EntitySet<T> implement IAsyncEnumerable<T> so that we can use ToListAsync() on EntitySet<T>

[axmty]

#394

In previous version of DataObjects, we could use AsAsync() on both IQueryable and EntitySet.
Now that AsAsync() has been removed, we expect ToListAsync() to work for both IQueryable and EntitySet types, but there is a runtime error since EntitySet does not implement IAsyncEnumerable.

[alex-kulakov]

I'm rejecting this PR. The idea is good, but many things haven't implemented to EntitySets be fully functional, the collection uses many APIs which also have to support async operations and it is hard to implement for someone outside the team.

Since root idea was to be able to use certain extensions with EntitySets, I just improved compatibility of those extensions with EntitySets (PR #413), which is way simpler than implementing full support of IAsyncEnumerable by EntitySet itself.