GitLab

Implement examples that show how to open a read-only blockstore, and a
read-write blockstore along with resumption from the same file.

The example surfaced a race condition where if the `AllKeysChan` is
partially consumed and file is closed then the gorutie started by chan
population causes the race condition. Locking on Close will make the
closure hang. Better solution is needed but choices are limited due to
`AllKeysChan` signature.

Fixes:
- https://github.com/ipld/go-car/issues/124