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.
Add hooks that contain protocol specific interfaces that can be used
to extend the protocol. This makes it possible to add new interfaces
to the protocol and implement the protocol specific data transport.
Move these protocol specific extension to the extensions directory.
