Add items for vacuum not having to wait for CONCURRENTLY, and
CONCURRENTLY not having to wait for other CONCURRENTLY operations.

Reported-by: Simon Riggs

Discussion: https://postgr.es/m/CANbhV-EMM4nf7Ys-Yae_kY25dXT_3eiOXke2+yw44jgy+4jNsA@mail.gmail.com

Backpatch-through: 14 only