Add alerting and runbook for logged subtransactions

We know subtransactions can trigger database slowdowns and should be avoided. Nearly all subtransactions have been removed from the application, but they can pop up from infrequently used codepaths. Develop process for alerting on these events and filing infradev issues to resolve them.

When subtransaction occurs in production:

• Alert the sre-oncall
• Runbook for filing infradev issue to remove codepath generating subtransactions