Fix naming issue and implement IAsyncDisposable

[Poker-sang]

Dowloader's api design is handy and makes sense! But there are some methods with a Task return value, named without using the Async suffix, which confuses me when I use them. The Async suffix is also used in some void methods, such as DowloadService.CancelAsync.

Also I would like DowloadService to use DisposeAsync instead of Clear so that the await using syntax can be used.

Thank you!