RUST-1399 Implement `AsyncRead` for `GridFsDownloadStream`

[isabelatkinson]

Implements the streaming API for GridFS downloads. Some relevant reading related to the async stuff going on here:

• This forum thread which discusses caching futures.
• tokio's implementation of the AsyncRead trait for File. This is tokio's AsyncRead rather than future's but demonstrates future caching and a similar state machine.

I'm going to add some unit tests to cover cases related to buffer size and reading into multiple buffers, but I wanted to request review on this now to get some feedback while I'm away.