Changes for item 6: changeset R,  :extend_alap option

Item 2 from google doc: Change Set R

Was agreed to adopt, BUT might need some additional discussion because I changed something:
In the original proposal, this option is optional to support by server, to make things easily backwards compatible with existing servers.
However, since the federation AM API is not backwards compatible (dropping the geni_ prefix among other things, means that new server code has to be written for it), I think it is to be decided if this option is mandatory to be supported by the server. If not, the server could just ignore the option. But as this is a useful and easy to implement option, it is probably best to make it mandatory for it to be supported by the server.

This pull request already assumes the option is mandatory.