af-binder: Fix of subscribe issue

af-binder: Fix of subscribe issue

Enforce serialization of messages, makes event really asynchronous,
and enforce (un)subscribe in living request.

Include the following changes:
 * 909893e afb-evt: Improve compatibility to guppy
 * 248ec5d afb-evt: send events in separate job
 * c8cc0d6 Revert "afb-stub-ws: Allow unordered process of messages"
 * bc38af3 afb-xreq: Forbids (un)subscribes after reply
 * 0342cd1 afb-client-demo: Allow pipe of messages

Bug-AGL: SPEC-2215
Bug-AGL: SPEC-2219
Bug-AGL: SPEC-2542
Bug-AGL: SPEC-2599

Change-Id: I357aeb852e2041ac6056dd563dac42f29dadb52d
Signed-off-by: José Bollo <jose.bollo@iot.bzh>