chore: relax a bunch of Sync requirements where unnecessary

[miraclx]

Linked: ferristseng/rust-ipfs-api#140

I noticed a weird trait restriction in https://github.com/ferristseng/rust-ipfs-api that made it so only streams implementing Sync are permitted to the upload API. Even though the download API doesn't abide by those same rules.

Effectively makes it so you can't pipe the output of, say.. cat into add. Because add expects more than cat can offer.

And this will be the case most of the time. I looked under the hood, and couldn't find any reasons why the stream itself needs to be Sync.

So this PR gets rid of all the Sync bounds where they are present.

I've also tested it downstream with the ipfs-api repo and in my own project as well, and it all checks out.

[ferristseng]

Thanks! This looks great.