Draft: Resolve "Celery required threads to be gevent patched"

Closes #3825

The idea is to patch Python threads by default in BLISS shell only. For code using BLISS as a library, or Flint, the behaviour is not changed from before: threads are not patched.

Still it is now possible to override the proposed behaviour by setting the BLISS_PATCH_THREADS environment variable prior to importing BLISS.