Add an Assert() to max_parallel_workers enforcement.

To prevent future bugs along the lines of the one corrected by commit
8ff51869, or find any that remain
in the current code, add an Assert() that the difference between
parallel_register_count and parallel_terminate_count is in a sane
range.

Kuntal Ghosh, with considerable tidying-up by me, per a suggestion
from Neha Khatri.  Reviewed by Tomas Vondra.

Discussion: http://postgr.es/m/CAFO0U+-E8yzchwVnvn5BeRDPgX2z9vZUxQ8dxx9c0XFGBC7N1Q@mail.gmail.com