Provide upgrade script for transition away from Meta MTA daemon

Summary:
When switching from using the MetaMTADaemon to a Taskmaster for sending mail,
if there are messages queued for delivery, they need to be re-queued into the
task system. This patch does that.

Task ID: #

Blame Rev:

Test Plan:
Ran it.

Revert Plan:

Tags:

Reviewers: epriestley, btrahan, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1780