Issue Summary
unack does change status to open if ack was not found in history

Environment

• OS: Linux
• API version: 9.0.4
• Deployment: Docker
• Database: Postgres
• web UI version: 8.7.1

To Reproduce
Steps to reproduce the behavior:

1. create new alert: r = api.send_alert(environment='Development', service=['service123'], resource='resource213', event='evtst', severity='minor', value=25)
2. ack the alert: api.action(r[1].id, 'ack', 'test ack')
3. send the alert to alerta 100 times with new value:

for i in range(100):
    r = api.send_alert(environment='Development', service=['service123'], resource='resource213',
                       event='evtst', severity='minor', value=i)

6. unack the alert: api.action(r[1].id, 'unack')

Expected result: status changed to open.
Actual result: status remains the same(ack).