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/psr/log/README.md

@@ -0,0 +1,58 @@
+PSR Log
+=======
+
+This repository holds all interfaces/classes/traits related to
+[PSR-3](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-3-logger-interface.md).
+
+Note that this is not a logger of its own. It is merely an interface that
+describes a logger. See the specification for more details.
+
+Installation
+------------
+
+```bash
+composer require psr/log
+```
+
+Usage
+-----
+
+If you need a logger, you can use the interface like this:
+
+```php
+<?php
+
+use Psr\Log\LoggerInterface;
+
+class Foo
+{
+    private $logger;
+
+    public function __construct(LoggerInterface $logger = null)
+    {
+        $this->logger = $logger;
+    }
+
+    public function doSomething()
+    {
+        if ($this->logger) {
+            $this->logger->info('Doing work');
+        }
+           
+        try {
+            $this->doSomethingElse();
+        } catch (Exception $exception) {
+            $this->logger->error('Oh no!', array('exception' => $exception));
+        }
+
+        // do something useful
+    }
+}
+```
+
+You can then pick one of the implementations of the interface to get a logger.
+
+If you want to implement the interface, you can require this package and
+implement `Psr\Log\LoggerInterface` in your code. Please read the
+[specification text](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-3-logger-interface.md)
+for details.