evdns: use evdns_base->global_requests_* instead of nameserver->active_requests

As Nick adviced. (#51)

nameserver->active_requests is used only with
EVDNS_BASE_DISABLE_WHEN_INACTIVE flag.
evdns: delete event for nameserver struct in request_finished().

As Nick notice, without this patch, with
EVDNS_BASE_DISABLE_WHEN_INACTIVE, the only way for a request to no
longer be active is by getting a reply (which is false: it can also
timeout, or get canceled, and I'm not sure those are the only ways).

Pull request #51