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 globally visible but the mail row is still isolated inside the transaction.
• (Connection A) COMMIT
• Now we're clear: the mail row is globally visible.

Change this code to reflect what's actually happening.

This means that if the worker row insert fails for some reason, we'll now throw with a mail row written to the database. But this is fine: it doesn't send on its own (so it can't cause mail loops or anything) and it can be re-queued with bin/mail resend if necessary without too much trouble.

Test Plan: See T12844 for particulars. Made some comments on tasks, saw the daemons send mail.

Reviewers: chad, amckinley, jmeador

Reviewed By: jmeador

Maniphest Tasks: T12844

Differential Revision: https://secure.phabricator.com/D18124