Allow disabling ad-hoc socket spawning

We'll need this for handling `IAudioProcessor` method calls in VST3. We
basically want a `Vst3MessageHandler` per `IAudioProcessor` instance,
but without the additional socket spawning or extra thread.

src/common/communication/vst3.h

@@ -38,9 +38,14 @@
  * @tparam Thread The thread implementation to use. On the Linux side this
  *   should be `std::jthread` and on the Wine side this should be `Win32Thread`.
  * @tparam Request Either `ControlRequest` or `CallbackRequest`.
+ * @tparam ad_hoc_sockets Whether new sockets should be created on demand to be
+ *   able to handle multiple function calls at the same time. If this is set to
+ *   false, then simultaneous `send_message()` calls will have to wait for the
+ *   earlier call to finish. This also means that the listening side does not
+ *   have to spawn a thread to constantly listen for new connections.
  */
-template <typename Thread, typename Request>
-class Vst3MessageHandler : public AdHocSocketHandler<Thread> {
+template <typename Thread, typename Request, bool ad_hoc_sockets>
+class Vst3MessageHandler : public AdHocSocketHandler<Thread, ad_hoc_sockets> {
    public:
     /**
      * Sets up a single main socket for this type of events. The sockets won't
@@ -59,7 +64,9 @@ class Vst3MessageHandler : public AdHocSocketHandler<Thread> {
     Vst3MessageHandler(boost::asio::io_context& io_context,
                        boost::asio::local::stream_protocol::endpoint endpoint,
                        bool listen)
-        : AdHocSocketHandler<Thread>(io_context, endpoint, listen) {}
+        : AdHocSocketHandler<Thread, ad_hoc_sockets>(io_context,
+                                                     endpoint,
+                                                     listen) {}
 
     /**
      * Serialize and send an event over a socket and return the appropriate
@@ -272,12 +279,12 @@ class Vst3Sockets : public Sockets {
      * This will be listened on by the Wine plugin host when it calls
      * `receive_multi()`.
      */
-    Vst3MessageHandler<Thread, ControlRequest> host_vst_control;
+    Vst3MessageHandler<Thread, ControlRequest, true> host_vst_control;
 
     /**
      * For sending callbacks from the plugin back to the host. After we have a
      * better idea of what our communication model looks like we'll probably
      * want to provide an abstraction similar to `EventHandler`.
      */
-    Vst3MessageHandler<Thread, CallbackRequest> vst_host_callback;
+    Vst3MessageHandler<Thread, CallbackRequest, true> vst_host_callback;
 };