Lot of changes here.

1. check_classic_mac_basic.yml — update to latest macOS-latest
2. docker-on-push-tag-or-pr — dependant updates
3. FFTConvolver — fix a compilation warning when the DEBUG flag is turned off
4. Reorganise some source files — e.g. move most of the metadata stuff into a subdirectory
5. Remove unused special-purpose AirPlay 2 RC event handler.
6. Reorganise source files and source code relating to the standard event handler.
7. Check configuration settings a little more carefully to offer more meaningful diagnostics.
8. Add password to AirPlay 2 and reorganise initialisation to set AirPlay 2 flags accordingly.  Also, remove redundant airplay_pin.
9. Remove redundant debug facility definitions from common.h
10. Make it possible to omit all the dacp stuff from a build.
11. Make it possible to omit metadata multicast from a build.
12. Add AP2-native metadata by setting bit 50 of features to true.
13. Improve stability when pre-empting an existing play session.
14. Fix a few compiler warnings in the pair_ap library.
15. Ensure that the audio format is known before starting synchronisation.
16. Improve handle_configure to set/clear a new config setting. Doesn’t seem to do anything…
17. Experimentally turn off bit 46 of AirPlay 2 features — seems to stop failing pair-verify attempts.

dacp.c

@@ -43,7 +43,7 @@
 #include <time.h>
 #include <unistd.h>
 
-#include "metadata_hub.h"
+#include "metadata/hub.h"
 #include "tinyhttp/http.h"
 
 typedef struct {
@@ -397,10 +397,10 @@ void relinquish_dacp_server_information(rtsp_conn_info *conn) {
   // as the conn's connection number
   // this is to signify that the player has stopped, but only if another thread (with a different
   // index) hasn't already taken over the dacp service
-  debug_mutex_lock(&dacp_server_information_lock, 500000, 2);
+  debug_mutex_lock(&dacp_server_information_lock, 500000, 4);
   if (dacp_server.players_connection_thread_index == conn->connection_number)
     dacp_server.players_connection_thread_index = 0;
-  debug_mutex_unlock(&dacp_server_information_lock, 3);
+  debug_mutex_unlock(&dacp_server_information_lock, 4);
 }
 
 // this will be running on the thread of its caller, not of the conversation thread...