Remove isc_task_sendto(anddetach) functions

The only place where isc_task_sendto() was used was in dns_resolver unit, where the "sendto" part was actually no-op, because dns_resolver uses bound tasks. Remove the isc_task_sendto() and isc_task_sendtoanddetach() functions in favor of using bound tasks create with isc_task_create_bound().

Additionally, cache the number of running netmgr threads (nworkers) locally to reduce the number of function calls.