chg: dev: Lazily allocate fetch counter

The counter in ns_client_t is used to track the maximum number of
recursions in the resolver, but it is created unconditionally when
starting the client and deallocated when resetting it.

This commit defers the allocation of the counter till recursion needs to
actually happen, speeding up authoritative workloads in perflab by
1.5~2%.

Merge branch 'alessio/lazy-fetch-counter-alloc' into 'main'

See merge request isc-projects/bind9!10917