ref(grouping): Do final `_save_aggregate_new` cleanup tasks

[lobsterkatie]

This does a last batch of cleanup in _save_aggregate_new before the new logic is added.

Notes:

• release is only used once, and can be pulled straight out of job, so it doesn't need to be passed separately.

• The logging about missing severity scores is leftover from the initial POC back in September, mostly just me being paranoid. We don't need it anymore.

• I nearly took the second check of the double-check lock out before I realized what it was doing there. Fortunately Markus name-dropped double-entrant locking in his PR description when he put it in. Since it's not immediately obvious why it's there, I added some explanation.

• The metrics.timer context manager's metric_tags was shadowing the metric_tags being passed in. Fixing that will let us use the latter tags in record_new_group_metrics if we ever want to.