Skip loading the rule's schedule after an alerting rule runs

[mikecote]

We previous loaded the alerting rule's schedule at the end of a run so we could update the task schedule if ever the rule was updated while the rule was running. This edge case comes at the cost of loading each rule a second time, causing a lot of unnecessary I/O at scale. Instead we should re-use the rule schedule from the rule that was loaded before the alerting rule ran. In the edge case scenario, the rule will take it's new schedule at the end of the next execution.

Definition of Done

• Rule is only loaded once at the beginning of a rule run
• Schedule is still returned at the end of a task run but from the rule object loaded at the beginning

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)