Ease pytest-xdist usage

[smarie]

As per suggestion from @larsks in #34: current support for pytest-xdist is still a bit complex: users have to follow the example [conftest.py`](https://smarie.github.io/python-pytest-harvest/#pytest-x-dist).

We could do several things to improve this :

• we could implement the pytest_sessionfinish hook with a "execute last" and if we are in a xdist worker, automatically persist all saved fixtures and report items - and if we are in the xdist master, automatically delete all saved fixtures and report items

• in the get_synthesis_dct base method, if we are the xdist master, we should retrieve previously persisted data and use it instead of the saved fixtures and report items

The final user experience would be that they can use the get_session_results_df() method in the pytest_sessionfinish hook, without caring about persistence and sharing across workers

[smarie]

@larsks this is now available in 1.9.0. I would appreciate any feedback about the new simplified user experience! See documentation