Destroy query in killoldestquery under a lock.
Fixes a race between ns_client_killoldestquery and ns_client_endrequest -
killoldestquery takes a client from recursing
list while endrequest
destroys client object, then killoldestquery works on a destroyed client
object. Prevent it by holding reclist lock while cancelling query.
(cherry picked from commit df3dbdff)
Closes #1650 (closed)