cluster/zeromq: Call DoTerminate() in destructor

Normal life-cycle is that Terminate() / DoTerminate() is called
by zeek-setup code. If that doesn't happen, shutdown and join
threads during destructor.

try { } catch (...) suggested by Benjamin.

src/cluster/backend/zeromq/ZeroMQ.h

@@ -20,6 +20,11 @@ public:
     ZeroMQBackend(std::unique_ptr<EventSerializer> es, std::unique_ptr<LogSerializer> ls,
                   std::unique_ptr<detail::EventHandlingStrategy> ehs);
 
+    /**
+     * Destructor.
+     */
+    ~ZeroMQBackend();
+
     /**
      * Spawns a thread running zmq_proxy() for the configured XPUB/XSUB listen
      * sockets. Only one node in a cluster should do this.