[9.20] chg: dev: Double the number of threadpool threads

Introduce this temporary workaround to reduce the impact of long-running
tasks in offload threads which can block the resolution of queries.

Related isc-projects/bind9#4898

Backport of isc-private/bind9!740

Merge branch 'backport-4898-workaround-double-threadpool-threads-9.20' into 'v9.20.2-release'

See merge request isc-private/bind9!741