feat(task): Record last success and failure run times in the Task

[brettbuddin]

Part of influxdata/flux#3080

We currently record the following "last run" information on the Task record:

• LatestCompleted (time)
• LatestScheduled (time)
• LastRunStatus (string)
• LastRunError (string)

This change introduces two new timestamps that reflect the last success and failure times of a Task (respectively):

• LatestSuccess (time)
• LatestFailure (time)

With these timestamps we will be able to compose new APIs for the Task system to make time-gap resilient (in the face of an arbitrary number of Task failures) easier to write. The next step will be to inject this information into Tasks as they get handed off to the Query layer to be run.

• CHANGELOG.md updated with a link to the PR)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• Documentation updated or issue created (provide link to issue/pr)

[gavincabbage]

LGTM- your e2e failure seemed intermittent so I restarted

[adrian-thurston]

Saw it got killed so I also restarted it, but same result.

ok  	github.com/influxdata/idpe/objectstore/metrics	1.030s
?   	github.com/influxdata/idpe/objectstore/mock	[no test files]
Killed
script returned exit code 137