fix:usr: Fix the data race causing a permanent active client increase

Previously, a data race could cause a newly created fetch context for a new client to be used
before it had been fully initialized, which would cause the query to become stuck; queries for the same
data would be either paused indefinitely or dropped because of
the `clients-per-query` limit. This has been fixed.

Closes #5053

Merge branch '5053-fetch-context-create-data-race' into 'main'

See merge request isc-projects/bind9!10146