Log Message:
dovecot2: update to 2.3.1

Changes:
* Submission server support improvements and bug fixes
  - Lots of bug fixes to submission server
* API CHANGE: array_idx_modifiable will no longer allocate space
 - Particularly affects how you should check MODULE_CONTEXT result, or use \ 
REQUIRE_MODULE_CONTEXT.

+ mail_attachment_detection_options setting controls when
  $HasAttachment and $HasNoAttachment keywords are set for mails.
+ imap: Support fetching body snippets using FETCH (SNIPPET) or
  (SNIPPET (LAZY=FUZZY))
+ fs-compress: Automatically detect whether input is compressed or not.
  Prefix the compression algorithm with "maybe-" to enable the
  detection, for example: "compress:maybe-gz:6:..."
+ Added settings to change dovecot.index* files' optimization behavior.
  See https://wiki2.dovecot.org/IndexFiles#Settings
+ Auth cache can now utilize auth workers to do password hash
  verification by setting auth_cache_verify_password_with_worker=yes.
+ Added charset_alias plugin. See
  https://wiki2.dovecot.org/Plugins/CharsetAlias
+ imap_logout_format and pop3_logout_format settings now support all of the \ 
generic variables (e.g. %{rip}, %{session}, etc.)