entitlement: fix bug in ProxyBlockingStateDao

If an entitlement is already cancelled (entry in blocking_states table),
don't add the blocking state cancellation event we deduced from subscriptions
(not needed and will be wrong).

This fixes test_cancel_ao_prior_future_bp_cancel_date in killbill-integration-tests.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Merge branch 'entitlement-bug-fixes' of github.com:killbill/killbill into entitlement-bug-fixes