Make annotations available in the thread_state

The following is a useful code that Dask distributed provides to get the key from a task, inside the task code:

```python
from distributed.worker import thread_state
task_key = thread_state.key
```

I would like to suggest making the annotations available using the same mechanism:

```python
from distributed.worker import thread_state
# Get the annotations, if defined.
annotations = thread_state.annotations
```

This is very important to facilitate the implementation of distributed tracing and log correlation using single request ids, provided via annotations.