session-manager: split policy and monitor

Separate the session manager in a monitor and policy part.

The monitor manages the devices and endpoints.

The policy watches the nodes/ports/clients and applies the policy
of linking them.

Because both now have a separate connection, we can remove some
hacks in the protocol. When a remote was both the implementer and
user of an object we could get in a deadlock when the user was
blocked waiting and the implementator was blocked sending a reply.
We used to un-busy a client when it was expecting a reply from a
ping or sync for this reason.

Add and use some more keys for the endpoints and streams.

src/modules/module-protocol-native/protocol-native.c

@@ -416,7 +416,6 @@ static void core_event_marshal_ping(void *object, uint32_t id, int seq)
 	struct spa_pod_builder *b;
 	struct pw_protocol_native_message *msg;
 
-	pw_client_set_busy(pw_resource_get_client(resource), false);
 	b = pw_protocol_native_begin_resource(resource, PW_CORE_PROXY_EVENT_PING, &msg);
 
 	spa_pod_builder_add_struct(b,