Add ParallelSlotSetIdle().

Add ParallelSlotSetIdle().

This commit refactors the code for marking a ParallelSlot as idle
to a new static inline function.  This can be used to mark a slot
that was obtained via ParallelSlotGetIdle() but that we don't
intend to actually use for a query as idle again.

This is preparatory work for a follow-up commit that will add a
--dry-run option to vacuumdb.

Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com
Discussion: https://postgr.es/m/CADkLM%3DckHkX7Of5SrK7g0LokPUwJ%3Dkk8JU1GXGF5pZ1eBVr0%3DQ%40mail.gmail.com