1096289 - Only start scheduler threads if lazy=False.

Multiple threads were being started when only should have been, and some were being started earlier than expected. This is a known celery behavior[0].  This could interfere with Celery's forking model of celerybeat, causing the traceback related to BZ 1096289   [1].

[0]. https://github.com/celery/celery/issues/1549
[1]. https://bugzilla.redhat.com/show_bug.cgi?id=1096289