Update chat/ejabberd to 16.09.

New XMPP Extension support
- Support for XEP-0355 - Namespace Delegation
- Support for XEP-0356 - Privileged Entity

Elixir support
- Support for Elixir-based configuration files

MUC/Sub
- Add a new request to list MUC room subscribers
- Link MUC subscription to bare JID
- New commands unsubscribe_room and unsubscribe_room for MUC
  subscriptions management through API
- Fix nick-to-jid mapping for MUC subscribers
- Fix subscribed rooms list retrieval
- Fix message routing from subscribers
- Do not update muc_online_users table on MUC/Sub operations

MUC
- Support multiple JIDs in room invitations
- Create room on configuration request as per XEP-0045, 10.1.3

PubSub
- Add SQL support for microblogging node plugin
- Fix publish with subscriber publish model and SQL backend
- node_mb: Call node_pep instead of node_hometree

Stream Management
- New Stream Management option: ack_timeout
- Fix Stream Management feature for the websocket connections
- Cancel timer when waiting for resumption

MAM and Offline
- Don't store messages of type "headline"
- Simplify "assume_mam_usage" option
- Honor hint for any non-"error" message
- Store announcements for offline users

Database
- Session backend: fix clean Redis table
- mod_shared_roster: Support SQL backend

ejabberdctcl admin tool
- ejabberdctl: do not force access rules check on register command
- ejabberdctl: do not force command line to pass a credential
- New commands unsubscribe_room and unsubscribe_room for MUC
  subscriptions management
- New command create_room_with_opts to create room with custom
  config

ejabberd API
- New commands unsubscribe_room and unsubscribe_room for MUC
  subscriptions management
- New command create_room_with_opts to create room with custom
  config
- Add X-Admin and basic auth header to CORS allowed headers in
  http_api
- Properly process OPTIONS header in http_api for all paths
- Expand parsing of JSON input to be able to handle update_roster
  command
- Provide proper args_desc in oauth_issue_token command

Developer features: ejabberd Hooks
- Add hooks c2s_session_pending, c2s_session_resumed,
  store_offline_message, store_mam_message
- Add CSI user's JID argument to csi_filter_stanza and
  csi_flush_queue hooks
- Add functions to get/set some c2s #state elements from external
  modules
- mod_client_state: Let other modules filter stanzas
- rework c2s API to simplify push modules integration

(fhajny)