Rewrite email parser using ImapEngine, harden processing loop

Replace webklex/php-imap with directorytree/imapengine in the ticket
email parser. ImapEngine is pure PHP over sockets.

Parser improvements:
- Wrap per-message processing in try/catch so one malformed email
  can't abort the run; failures are flagged and logged with UID
- Query unseen + unflagged so previously-failed (flagged) messages
  are no longer re-processed on every cron run
- Skip vacation/auto-responder emails (RFC 3834) to prevent mail
  loops with the ticket auto-reply
- Cap messages per run (50) and attachment size (15MB); inline
  images over 2MB are stored as attachments instead of base64-embedded
  in ticket details
- Atomic lock file creation
- preg_quote() the ticket prefix in subject matching
- Dedupe CC watchers and exclude the sender
- Map webklex 'tls' encryption setting to STARTTLS for compatibility

NDR/DSN parsing now walks MIME parts via the underlying
zbateson parser instead of relying on attachment extraction.

plugins/vendor/ralouphie/getallheaders/README.md

@@ -0,0 +1,27 @@
+getallheaders
+=============
+
+PHP `getallheaders()` polyfill. Compatible with PHP >= 5.3.
+
+[![Build Status](https://travis-ci.org/ralouphie/getallheaders.svg?branch=master)](https://travis-ci.org/ralouphie/getallheaders)
+[![Coverage Status](https://coveralls.io/repos/ralouphie/getallheaders/badge.png?branch=master)](https://coveralls.io/r/ralouphie/getallheaders?branch=master)
+[![Latest Stable Version](https://poser.pugx.org/ralouphie/getallheaders/v/stable.png)](https://packagist.org/packages/ralouphie/getallheaders)
+[![Latest Unstable Version](https://poser.pugx.org/ralouphie/getallheaders/v/unstable.png)](https://packagist.org/packages/ralouphie/getallheaders)
+[![License](https://poser.pugx.org/ralouphie/getallheaders/license.png)](https://packagist.org/packages/ralouphie/getallheaders)
+
+
+This is a simple polyfill for [`getallheaders()`](http://www.php.net/manual/en/function.getallheaders.php).
+
+## Install
+
+For PHP version **`>= 5.6`**:
+
+```
+composer require ralouphie/getallheaders
+```
+
+For PHP version **`< 5.6`**:
+
+```
+composer require ralouphie/getallheaders "^2"
+```