Prevent duplicated emails with send-immedialtely = true and MTA daemon running

Test Plan:
Set 'metamta.send-immediately' to true. Start up several MTA daemons, without
the patch you'll probably get multiple emails, with the patch you should get
only one.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, mareksapota, epriestley

Differential Revision: 1021