Moved TaskHelpers into the new FoundationDB.Async namespace
Added a new infrastructure to process async tasks in parallel [WiP]

- IAsyncTarget<T> is an async version of a IObserver<T>, but that can rate limit the producer if it is full
- IAsyncSource<T> can return processed values when they are ready (rate limiting the consumer)
- IAsyncPump<T> is for pumps, that get items from an IAsyncSource<T> and push them to an IAsyncTarget<T>
- Maybe<T> is the "message" that holds a value, an error, or empty to signify that the sequence is complete
- Added an implementation of AsyncBuffer that just holds N items (with optionnal inline transformation)
- Added an implementatino of AsyncTaskBuffer, that holds N tasks and outputs their result when they complete (in order of arrival or completion)
Added some async helpers

- PumpToListAsync : pump the content of a source into a list
- TransformToListAsync: transform the items in a source in parallel, and outputs the results (in order or arrival) into a list
- added unit tests