Inconsistent timings in `job.status` vs `task_jobs` DB table

[MetRonnie]

Description

For a workflow with a polling task, we have seen inconsistencies between the job.status file and the DB:

Time	job.status	db (task_jobs)
submit time	N/A[^1]	16:27:38
submit exit time	16:27:41	16:27:40
init / run time	16:27:51	16:29:51
exit time	16:29:51	16:29:51

[^1]: I think the "submit time" is when the scheduler started the submission process, whereas the "submit exit time" is when the job was actually submitted or submit-failed in the job runner.

[MetRonnie]

When using the poll comms method, if the started output is missed because it last polled as submitted and then next polled as succeeded, the started time is wrongly reported as the time of polling the success

https://github.com/cylc/cylc-flow/blob/a721f852f75c20e39e5f4ebba303f9af56c89ed9/cylc/flow/task_events_mgr.py#L1354-L1364