`NeptuneCallback` produces lots of `X-coordinates (step) must be strictly increasing` errors

Nobody has claimed this yet.

### Bug description

When Optuna is run in parallel mode (`n_jobs=-1`), with `NeptuneCallback`, I get:
`[neptune] [error ] Error occurred during asynchronous operation processing: X-coordinates (step) must be strictly increasing for series attribute: trials/values. Invalid point: 0.0`
It's normal that during parallel or distributed hyperparam optimization, information become unordered. Either Neptune should support adding steps out of order, or `NeptuneCallback` should support it somehow (e.g. by using an artificial step number).

### What version are you seeing the problem on?

v1.x

### How to reproduce the bug

```python
study.optimize(..., callbacks=[NeptuneCallback(run)], n_jobs=-1)
```

### Error messages and logs

`[neptune] [error ] Error occurred during asynchronous operation processing: X-coordinates (step) must be strictly increasing for series attribute: trials/values. Invalid point: 0.0`

### Environment

Any multi-threaded environment.

### More info

_No response_

Start at the NeptuneCallback entry point and reproduce the issue with study.optimize(..., callbacks=[NeptuneCallback(run)], n_jobs=-1). Check how parallel trials are recorded in the trials/values series; done means the multi-threaded example no longer produces strictly increasing step errors.

Written by the indexing model from the issue text.