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/http-message/CHANGELOG.md

@@ -0,0 +1,36 @@
+# Changelog
+
+All notable changes to this project will be documented in this file, in reverse chronological order by release.
+
+## 1.0.1 - 2016-08-06
+
+### Added
+
+- Nothing.
+
+### Deprecated
+
+- Nothing.
+
+### Removed
+
+- Nothing.
+
+### Fixed
+
+- Updated all `@return self` annotation references in interfaces to use
+  `@return static`, which more closelly follows the semantics of the
+  specification.
+- Updated the `MessageInterface::getHeaders()` return annotation to use the
+  value `string[][]`, indicating the format is a nested array of strings.
+- Updated the `@link` annotation for `RequestInterface::withRequestTarget()`
+  to point to the correct section of RFC 7230.
+- Updated the `ServerRequestInterface::withUploadedFiles()` parameter annotation
+  to add the parameter name (`$uploadedFiles`).
+- Updated a `@throws` annotation for the `UploadedFileInterface::moveTo()`
+  method to correctly reference the method parameter (it was referencing an
+  incorrect parameter name previously).
+
+## 1.0.0 - 2016-05-18
+
+Initial stable release; reflects accepted PSR-7 specification.