pgsql: 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(dot)huinker(at)gmail(dot)com>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com
Discussion: https://postgr.es/m/CADkLM%3DckHkX7Of5SrK7g0LokPUwJ%3Dkk8JU1GXGF5pZ1eBVr0%3DQ%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/750816971b35270971dd77f37ed1e9655fce4ed9

Modified Files
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src/fe_utils/parallel_slot.c | 6 ++----
src/include/fe_utils/parallel_slot.h | 7 +++++++
2 files changed, 9 insertions(+), 4 deletions(-)