Move functionality of PhabricatorMetaMTADaemon to a worker task

Summary:
This will allow sending mail to be done by task workers. See T750.

Task ID: #

Blame Rev:

Test Plan:
- started taskmaster daemon in test env
- used "send new test message" feature in MetMTA (with send now unchecked)
- confirmed receipt of 1 email
- repeated 2 & 3 with send now checked

Revert Plan:

Tags:

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T388, T750

Differential Revision: https://secure.phabricator.com/D1723

src/docs/configuration/configuring_outbound_email.diviner

@@ -133,18 +133,15 @@ disable outbound mail, if you don't want to send mail or don't want to configure
 it yet. Just set **metamta.mail-adapter** to
 "PhabricatorMailImplementationTestAdapter".
 
-= Configuring the MetaMTA Daemon =
+= Configuring MetaMTA to Send Mail Using a Daemon =
 
 Regardless of how you are sending outbound email, you can move the handoff to
 the MTA out of the main process and into a daemon. This will greatly improve
 application performance if your mailer is slow, like Amazon SES. In particular,
 commenting on Differential Revisions and Maniphest Tasks sends outbound email.
 
-To use the MetaMTA daemon:
-
-  - set **metamta.send-immediately** to ##false## in your configuration; and
-  - launch a ##metamta## daemon with ##phabricator/bin/phd launch metamta##.
-
+If you set **metamta.send-immediately** to ##false## in your configuration,
+MetaMTA will queue mail to be send by a PhabricatorTaskmasterDaemon.
 For more information on using daemons, see @{article:Managing Daemons with phd}.
 
 = Testing and Debugging Outbound Email =
@@ -169,4 +166,4 @@ Continue by:
   - @{article:Configuring Inbound Email} so users can reply to email they
   receive about revisions and tasks to interact with them; or
   - learning about daemons with @{article:Managing Daemons with phd}; or
-  - returning to the @{article:Configuration Guide}.
+  - returning to the @{article:Configuration Guide}.

src/docs/configuration/managing_daemons.diviner

@@ -65,8 +65,6 @@ You can get a list of launchable daemons with **phd list**:
 
   - **libphutil test daemons** are not generally useful unless you are
     developing daemon infrastructure or debugging a daemon problem;
-  - **PhabricatorMetaMTADaemon** sends mail in the background, see
-    @{article:Configuring Outbound Email} for details;
   - **PhabricatorTaskmasterDaemon** runs a generic task queue; and
   - **PhabricatorRepository** daemons track repositories, descriptions are
     available in the @{article:Diffusion User Guide}.