entitlement: fix bug in EventsStream

In some scenarii (e.g. IMM billing / EOT entitlement cancellations or EOT changes),
the future add-on entitlement cancellations weren't seen.

This fixes test_cancel_bp_with_ent_eot_bill_imm in killbill-integration-tests.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>