Expose some way of asynchronous blocking lock

Is there a way to make lock blocking, or at least asynchronously blocking?

So one could do:

```
await lock(fd, timeout)
```

Seems like there are 3 possible situations:

1. Don't block at all, return false
2. Block for a certain amount of time like ms
3. Block forever

Would this require some C++ changes?

Start by reviewing the existing lock API and how it handles file descriptors, then assess whether `await lock(fd, timeout)` can support the three requested blocking modes. Clarify the timeout semantics and whether C++ changes are required; done requires an agreed implementation scope and tests for each mode.

Written by the indexing model from the issue text.