Work on the Server subscriptions system (#30)

* Rework the Client to support multiple concurrent requests

* Restore proc-macro functionnality

* More work

* More work

* Work on the Server subscriptions system

* More work on design

* More work

* More work

* Even more work

* Subscriptions working

* Dix docs

* More test fix

* Fix server-sent subscription notifications

* Add some server tests

* Subscriptions pending system working

* Remove TODO from tests

* Implement close() correctly

* Add comment on close()

* Fix proc macro

* Fix doctest

* Fix local doctest