fix: nil: Allocate work threads from their owning loop's memory context

The per-loop worker threads allocated their state from the loop manager's
memory context instead of the per-loop context that owns them. Allocate
from the owning loop's context and hold a reference to that loop for the
thread's lifetime, matching the context handling already used on the
work-enqueue and completion paths.

Already changed as part of 9.20 backport.

Merge branch 'ondrej/rewrite-threadpool-fixups' into 'main'

See merge request isc-projects/bind9!12268