multiprocessing support

We have had multiple reports of problems using zarr-python 3.x in a multiprocessing context: #2729, https://ossci.zulipchat.com/#narrow/channel/423692-Zarr/topic/Zarr.20v3.20force.20single.20threaded.20for.20forked.20processes. 

We should:
- ensure that our array / group classes play nicely in a multiprocessing context
- ensure that multiprocessing support is properly tested in the test suite