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/composer.json

@@ -0,0 +1,26 @@
+{
+	"name": "ralouphie/getallheaders",
+	"description": "A polyfill for getallheaders.",
+	"license": "MIT",
+	"authors": [
+		{
+			"name": "Ralph Khattar",
+			"email": "ralph.khattar@gmail.com"
+		}
+	],
+	"require": {
+		"php": ">=5.6"
+	},
+	"require-dev": {
+		"phpunit/phpunit": "^5 || ^6.5",
+		"php-coveralls/php-coveralls": "^2.1"
+	},
+	"autoload": {
+		"files": ["src/getallheaders.php"]
+	},
+	"autoload-dev": {
+		"psr-4": {
+			"getallheaders\\Tests\\": "tests/"
+		}
+	}
+}