Alert executors are not provided the date and time of the last successful execution

[mikecote]

The previousStartedAt keeps changing when the previous execution failed. The documentation states this should be the last successful execution.

Steps to reproduce:

• Add a log for the previousStartedAt value and then a throw new Error('Fail'); line here
• Create an index threshold alert that runs every 10 seconds
• Notice previousStartedAt keeps increasing but should represent the last successful execution

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[gmmorris]

Given this is used by SIEM to set the timeframe of their detection queries, we think this might be high impact.
Adding this to the GA queue.