cluster: Add SubscribeCallback support

This allows callers of Subscribe() to pass in a callback that will be invoked
once the subscription is established or failed to establish. It is the
backend's responsibility to execute the callback on the main thread either
synchronously, or preferably asynchronously at a later point, by
scheduling a task on the IO main loop.

This turns on ZMQ_XPUB_VERBOSE for ZeroMQ so that notifications about
subscriptions are raised even if the subscriptions has previously been
observed.

src/cluster/Backend.h

@@ -185,13 +185,40 @@ public:
         return DoPublishEvent(topic, event);
     }
 
+    /**
+     * Status codes for callbacks.
+     */
+    enum class CallbackStatus {
+        Success,
+        Error,
+        NotImplemented,
+    };
+
+    /**
+     * Information for subscription callbacks.
+     */
+    struct SubscriptionCallbackInfo {
+        CallbackStatus status;              // The status of the operation.
+        std::optional<std::string> message; // Optional message.
+    };
+
+    using SubscribeCallback =
+        std::function<void(const std::string& topic_prefix, const SubscriptionCallbackInfo& info)>;
+
     /**
      * Register interest in messages that use a certain topic prefix.
      *
+     * Invoking cb may happen while Subscribe() executes, for example if the
+     * call to Subscribe() is synchronous, or an error is discovered before
+     * submitting any work.
+     *
      * @param topic_prefix a prefix to match against remote message topics.
+     * @param cb callback invoked when the subscription was processed.
      * @return true if it's a new event subscription and it is now registered.
      */
-    bool Subscribe(const std::string& topic_prefix) { return DoSubscribe(topic_prefix); }
+    bool Subscribe(const std::string& topic_prefix, SubscribeCallback cb = SubscribeCallback()) {
+        return DoSubscribe(topic_prefix, std::move(cb));
+    }
 
     /**
      * Unregister interest in messages on a certain topic.
@@ -314,13 +341,19 @@ private:
      * Register interest in messages that use a certain topic prefix.
      *
      * If the backend hasn't yet established a connection, any subscriptions
-     * should be queued until they can be processed.
+     * should be queued until they can be processed. If a callback is given,
+     * it should be called once the subscription can be determined to be
+     * active. The callback has to be invoked from Zeek's main thread. If
+     * the backend does not implement callbacks, it should invoke the callback
+     * with CallbackStatus::NotImplemented, which will act as success, but
+     * provides a way to distinguish behavior.
      *
      * @param topic_prefix a prefix to match against remote message topics.
+     * @param cb callback to invoke when the subscription is active
      *
      * @return true if it's a new event subscription and now registered.
      */
-    virtual bool DoSubscribe(const std::string& topic_prefix) = 0;
+    virtual bool DoSubscribe(const std::string& topic_prefix, SubscribeCallback cb) = 0;
 
     /**
      * Unregister interest in messages on a certain topic.