Pass pointers to IMessage objects around

Instead of serializing the actual `YaMessage`, for the reasons mentioned
in the comments. This was needed to stop iZotope VocalSynth 2 in Ardour
from segfaulting when editing parameters, because that plugin is
apparently being very naughty.

src/plugin/bridges/vst3.cpp

@@ -110,7 +110,7 @@ Vst3PluginBridge::Vst3PluginBridge()
                     -> YaConnectionPoint::Notify::Response {
                     return plugin_proxies.at(request.instance_id)
                         .get()
-                        .connection_point_proxy->notify(&request.message);
+                        .connection_point_proxy->notify(&request.message_ptr);
                 },
                 [&](const YaHostApplication::GetName& request)
                     -> YaHostApplication::GetName::Response {