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.
