media-session: deal with global id race conditions

To resolve monitor and policy core global ids racing with each other,
use separate registry event handlers for both cores.  Each handles only
their own objects, determined by where the object handle was created.

Postpone handling of policy core new global events after monitor sync,
which orders them after the corresponding monitor proxy and registry
events. Monitor core is then more up-to-date, so we resolve id clashes
in favor of monitor globals, which avoids duplicate objects.

Fix use-after-free by tracking whether a monitor holds references to
sm_object. Keep also objects pending for id in a list, so that they can
be cleaned up on session_shutdown (monitors may leak objects at
shutdown, because spa objectinfo events won't be handled then).

Caveats:

Zombie objects may still created if policy core is late by several
events, but in those cases the corresponding remove events are already
in the queue.

Also, there's a (theoretical) possibility that pw_registry_bind will
bind the wrong object, if the registry event is handled too late and an
id is reused by the server.

For details, see reverted 77e4fdb1e4
for which this is a another approach.

src/examples/media-session/bluez-monitor.c

@@ -302,8 +302,8 @@ static void bluez_device_event(void *data, const struct spa_event *event)
 				pw_node_set_param((struct pw_node*)node->snode->obj.proxy,
 					SPA_PARAM_Props, 0, props);
 			} else {
-				pw_log_warn("device %p: node %d not ready for volume yet",
-						device, id);
+				pw_log_warn("device %p: node %d object:%p not ready for volume yet",
+						device, id, node->snode);
 			}
 		}
 		break;