Access metadata.api only after `api` has exited the context and
thus the event loop has stopped and all incoming messages, like
the one setting the metadata, have been processed.
See commit 803433fb62 for a lecture
about the internals and all the details involved.
Simple check for the new server side method, `get-arguments`, and
client side counterpart, `api.arguments`, that compares that using
the later we get the supplied input (arguments) to API.
Now that jsoncomm is using a connection oriented protocol, the
`addr` parameter is not needed[*] and can thus be removed from
the `BaseAPI._message` message dispatcher. Adapt all usages
of it, including the tests.
[*] sendmsg ignores the destination parameter for connection
oriented sockets.
Add a new abstract class property to `BaseAPI` called `endpoint`,
meant to be implemented by deriving classes in order to identify
the end point name for the API provider.
Implement the new property in all existing API providers.
