Use atomics to avoid locking in InjectionPointRun()

Use atomics to avoid locking in InjectionPointRun()

This allows using injection points without having a PGPROC, like early
at backend startup, or in the postmaster.

The injection points facility is new in v17, so backpatch there.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Disussion: https://www.postgresql.org/message-id/4317a7f7-8d24-435e-9e49-29b72a3dc418@iki.fi