Enriched the extension helpers on async result

[zhenlineo]

Added a few extension methods for handling async result, such as SingleAsync, ForeachAsync.

[zhenlineo]

@ali-ince I removed the extension method
Task ForEachAsync(..., Function<IRecord, Task>)
but instead added
Task<List<T>> ToListAsync<T>(..., Function<IRecord, T>)

The reason I changed is that I am not sure if we should make decisions for a user to await on their tasks. Instead I think it might be better to give ToListAsync method where a user could return his/her tasks into a list<Task> and then waitAll on them.

And in the future, when requested, we could add
Task ForEachAsync(..., Function<IRecord, Task>) and
Task ForEachAsync<T>(..., Function<IRecord, Task<T>>)

[zhenlineo]

This code shows a "normal" usecase of Task<List<T>> ToListAsync(..., Func<IRecord, T>)