Make RandomAccess returned owned futures

[cowlicks]

A'll these trait methods return futures that borrow &mut self. This makes doing anything other than imperative one-after-another calls to methods almost impossible. So the trait was rewritten to return owned futures. Except for len() is is_empty() which are now sync and infallible.

For more information see this issue in the hypercore repo.

• decide what type data should be in RandomAccess.write

[cowlicks]

To whom it may concern:

RandomAccess.write's data parameter is under consideration.

The write method currently takes &[u8], but since BoxFuture is 'static the data must either be cloned before being moved into the future, or the caller must ensure it lives long enough. We haven't settled on the best owned type for this parameter. Candidates are Vec<u8> (no new dependency, universal), Bytes (cheaply cloneable, idiomatic in async I/O) and keeping &[u8] with a lifetime-bound future (zero-copy but no longer 'static). Feedback welcome.

I'm leaning towards using Bytes but I will decide that after publishing this as an x.x.x-alpha version.