Make ZMQ notification interface instance global.

This moves the used instance of CZMQNotificationInterface from a static
variable in init.cpp to a globally-accessible one declared in
zmq/zmqnotificationinterface.h.  The variable is also renamed to
g_zmq_notification_interface, to be consistent with other globals.

We need this to implement a new RPC method "getzmqnotifications" (see
https://github.com/bitcoin/bitcoin/issues/13526) in a follow up.

src/init.cpp

@@ -99,10 +99,6 @@ void DummyWalletInit::AddWalletOptions() const
 const WalletInitInterface& g_wallet_init_interface = DummyWalletInit();
 #endif
 
-#if ENABLE_ZMQ
-static CZMQNotificationInterface* pzmqNotificationInterface = nullptr;
-#endif
-
 #ifdef WIN32
 // Win32 LevelDB doesn't use filedescriptors, and the ones used for
 // accessing block files don't count towards the fd_set size limit
@@ -279,10 +275,10 @@ void Shutdown()
     g_wallet_init_interface.Stop();
 
 #if ENABLE_ZMQ
-    if (pzmqNotificationInterface) {
-        UnregisterValidationInterface(pzmqNotificationInterface);
-        delete pzmqNotificationInterface;
-        pzmqNotificationInterface = nullptr;
+    if (g_zmq_notification_interface) {
+        UnregisterValidationInterface(g_zmq_notification_interface);
+        delete g_zmq_notification_interface;
+        g_zmq_notification_interface = nullptr;
     }
 #endif
 
@@ -1409,10 +1405,10 @@ bool AppInitMain()
     }
 
 #if ENABLE_ZMQ
-    pzmqNotificationInterface = CZMQNotificationInterface::Create();
+    g_zmq_notification_interface = CZMQNotificationInterface::Create();
 
-    if (pzmqNotificationInterface) {
-        RegisterValidationInterface(pzmqNotificationInterface);
+    if (g_zmq_notification_interface) {
+        RegisterValidationInterface(g_zmq_notification_interface);
     }
 #endif
     uint64_t nMaxOutboundLimit = 0; //unlimited unless -maxuploadtarget is set

src/zmq/zmqnotificationinterface.cpp

@@ -180,3 +180,5 @@ void CZMQNotificationInterface::BlockDisconnected(const std::shared_ptr<const CB
         TransactionAddedToMempool(ptx);
     }
 }
+
+CZMQNotificationInterface* g_zmq_notification_interface = nullptr;

src/zmq/zmqnotificationinterface.h

@@ -37,4 +37,6 @@ private:
     std::list<CZMQAbstractNotifier*> notifiers;
 };
 
+extern CZMQNotificationInterface* g_zmq_notification_interface;
+
 #endif // BITCOIN_ZMQ_ZMQNOTIFICATIONINTERFACE_H