Make unread async

[make-github-pseudonymous-again]

This would allow compatibility with optimized on disk implementation that treats .unread(readToken) as a rewind operation. This is only really interesting if unread is called many times in a row.

[make-github-pseudonymous-again]

Although note that this can be done synchronously via a counter. So maybe not necessary. Needs maturity.

[make-github-pseudonymous-again]

Boundary checks may require asynchronicity.