afb-evt: Refactor processing of broadcasted events

Solving the bug SPEC-2625 needs to rework the broadcasting
of events. It appeared that the numerical event identifier
passed for broadcast wasn't used by called function except
for hooking. Suppressing it introduces a clear distinction
between the push and the broadcast paths.

The file afb-ws-json1 is changed to avoid casting of
functions.

Bug-AGL: SPEC-2625

Change-Id: I9fe75adc8086812b21b70ce28baffcf77bd5e1cf
Signed-off-by: José Bollo <jose.bollo@iot.bzh>