interfaces: improve remote API

Add return values to events and method callbacks. This makes it
possible to pass any error or async return value.
Add sync/done/error in both directions so that both proxy and resource
and perform/reply sync and produce errors.
Return a SPA_ASYNC from remote method calls (and events), keep the
sequence number in the connection.
With the core sync/done we can remove the client-node done method and
it's internal sequence number along with the seq number in method calls.
We can also use the method/event async return value to perform a sync
with as the unique sequence number for this method.
Add sync method and done/error event to proxy and resource.

src/modules/module-protocol-native/connection.h

@@ -90,7 +90,7 @@ struct spa_pod_builder *
 pw_protocol_native_connection_begin_proxy(struct pw_protocol_native_connection *conn,
                                           struct pw_proxy *proxy,
                                           uint8_t opcode);
-void
+int
 pw_protocol_native_connection_end(struct pw_protocol_native_connection *conn,
                                   struct spa_pod_builder *builder);