Clear websocket and mqtt connections on shutdown

This was previously implicitly done by a settings reload on shutdown,
but this has since been removed, and we need to clear the connections
explicitly.

plugins/base/utils/connection-manager.cpp

@@ -23,6 +23,7 @@ bool setup()
 {
 	AddSaveStep(saveConnections);
 	AddLoadStep(loadConnections);
+	AddPluginCleanupStep([]() { connections.clear(); });
 	return true;
 }
 

plugins/mqtt/mqtt-helpers.cpp

@@ -718,6 +718,7 @@ static bool setup()
 {
 	AddSaveStep(SaveMqttConnections);
 	AddLoadStep(LoadMqttConnections);
+	AddPluginCleanupStep([]() { GetMqttConnections().clear(); });
 	return true;
 }