Pin dns_request to the associated loop

When dns_request was canceled via dns_requestmgr_shutdown() the cancel event would be propagated on different loop (loop 0) than the loop where request was created on. In turn this would propagate down to isc_netmgr where we require all the events to be called from the matching isc_loop.

Pin the dns_requests to the loops and ensure that all the events are called on the associated loop. This in turn allows us to remove the hashed locks on the requests and change the single .requests list to be a per-loop list for the request accounting.

Closes #4086 (closed)