Fix: only one subscription could be made

Fix: only one subscription could be made

Only one subscription could be made and then only one subscriber
could receive associated events. As the relationship between events
and subscriber is not 1-1 but 1-N, so this was the mistake. Now 1 event
is created for N subscriber where before there was 1 event for 1
subscriber and subsequent subscribers could not receive same events
than the ones already subscribed.

Bug-AGL: SPEC-2726
Bug-AGL: SPEC-2976

Change-Id: I4e4a80ac9f9b6b4c6b006108ee55f355a7c1e377
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>