Detect main thread and adapt Blosc usage automatically

[alimanfoo]

Currently if you switch between using Zarr in the main thread (e.g., making Zarr API calls directly in an IPython session) and using from multi-threaded context (e.g., if you use a Zarr array as part of a Dask computation), you have to manually switch the way Blosc is used by calling zarr.blosc.use_context(True) and zarr.blosc.use_context(False). This is cumbersome for interactive analysis. This could be avoided if the Blosc extension checked if the current thread is the main thread, and if so use Blosc in non-contextual mode, otherwise use Blosc in contextual mode, so the user doesn't have to do any manual switching.