Ticket 48898 - Crash during shutdown if nunc-stans is enabled
Bug Description: During server shutdown the connection table was
freed before we shut down the nunc-stans thread
pool. This lead to a crash when a freed connection
pointer was deferenced in ns_handle_closure.
Fix Description: Close down the nunc-stans thread pool before freeing
the connection table.
Reviewed by: nhosoi(Thanks!)
(cherry picked from commit 02d8e0c0fc91750d2114b05afebdd457e4a04adf)