Use unordered maps for VST3 plugin instances

The better algorithmic time complexity should help when using many (say,
hundreds) of instances of a single VST3 plugin.

src/common/communication/common.h

@@ -744,7 +744,7 @@ class AdHocSocketHandler {
 
         // This works the exact same was as `active_plugins` and
         // `next_plugin_id` in `GroupBridge`
-        std::map<size_t, Thread> active_secondary_requests{};
+        std::unordered_map<size_t, Thread> active_secondary_requests{};
         std::atomic_size_t next_request_id{};
         std::mutex active_secondary_requests_mutex{};
         accept_requests(

src/common/communication/vst3.h

@@ -526,7 +526,8 @@ class Vst3Sockets final : public Sockets {
      * would have one dedicated thread for handling function calls to these
      * interfaces, and then another dedicated thread just idling around.
      */
-    std::map<size_t, Vst3MessageHandler<Thread, AudioProcessorRequest>>
+    std::unordered_map<size_t,
+                       Vst3MessageHandler<Thread, AudioProcessorRequest>>
         audio_processor_sockets;
     std::mutex audio_processor_sockets_mutex;
 };