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.
80 jobs for !3146 with 1650-fix-race-in-killoldestclient in 30 minutes and 27 seconds (queued for 6 seconds)
merge request