lib/dns/resolver.c: Convert (dns_view_t *)->weakrefs to isc_refcount_t

lib/dns/resolver.c: Convert (dns_view_t *)->weakrefs to isc_refcount_t

There's a deadlock in BIND 9 code where (dns_view_t){ .lock } and
(dns_resolver_t){ .buckets[i].lock } gets locked in different order.  When
view->weakrefs gets converted to a reference counting we can reduce the locking
in dns_view_weakdetach only to cases where it's the last instance of the
dns_view_t object.

(cherry picked from commit a7c9a52c89146490a0e797df2119026a268f294c)
(cherry picked from commit 232140edae6b80f8344db234cda28f8456269a6e)