Implement Split query functionality

[garywoodfine]

          Hey not to pile on, but if you're making changes, what do you think about adding support for AsSplitQuery?

I was thinking it might just be another optional boolean on the method like, and then conditionally tacking it on to the query:

public async Task<T> SingleOrDefaultAsync(....bool asSplitQuery = false) {
    ...
    if (asSplitQuery) query = query.AsSplitQuery();
    ...
}

But I have not had a chance to actually test it. Could always do it later as a non-breaking change too. Just thought I'd mention it.

Thanks again-

Originally posted by @KennethScott in #67 (comment)