Add mutexes for the VST3 data caches

Since the restart will always be called from another thread, and when a
plugin asks for a restart during initialization this can quickly lead to
issues.

src/plugin/bridges/vst3-impls/plugin-proxy.h

@@ -487,6 +487,7 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
      *       does that.
      */
     std::optional<BusInfoCache> processing_bus_cache;
+    std::mutex processing_bus_cache_mutex;
 
     /**
      * A cache for `IEditController::getParameterCount()` and
@@ -513,4 +514,5 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
      *       information four times per second.
      */
     ParameterInfoCache parameter_info_cache;
+    std::mutex parameter_info_cache_mutex;
 };