R1:3d70db9eb5d0

Queue a worker task to send mail only after committing the mail transaction

Summary:
Fixes T12844. This code is misleading: the daemon insert is happening on a different connection, and is not inside the transaction on the Mail connection.

What actually happens is this:

- (Connection A) `BEGIN`
- (Connection A) `INSERT INTO mail ...`
- (Connection B) `INSERT INTO worker ...` <-- This is a different connection, and it is NOT in a transaction!
- There's a race window here: the worker row is…