Update chat/ejabberd to 17.08.

Core
- Erlang/OTP 17.5 or higher is required, and 20 is now supported
- Make ejabberd_cluster modular
- Replace gen_fsm with p1_fsm to avoid warnings in OTP20+
- Fix clustering table reg_users_counter
- ext_mod: Update spec from custom and allow modules dependencies
- extauth.py: Fix to support : in passwords
- Set high water mark in lager for all backends
- Fix old route record in mnesia窶冱 route table haven窶冲 been remove
  when restarting in some cases
- ejabberd_cluster*.erl: Add copyright and fix description
- Add support of rfc6120 section 4.9.3.16 on node shutdown

Configuration
- ejabberd_c2s: Fix priority of 窶歪ertfile窶� option
- Introduce 窶鷲osts窶� modules option
- Fix ERLANG_OPTS, INET_DIST_INTERFACE and FIREWALL_WINDOW option
- Remove unused 窶藁anagers窶� option, related to the deferred XEP-0321

Commands
- Fix errors when running ejabberdctl as root
- Fix set_presence command to work in recent ejabberd
- Rename stop_all_connections to stop_s2s_connections for consistency
- Change policy of user_resources command, from user to admin
- Remove old command calling interface
- Describe more command arguments and results

Modules
- mod_http_api: Use hide_sensitive_log_data option when registering
  users
- mod_http_fileserver: Request basic auth dialog from browser
- mod_muc: Fix nick bug with MUC on riak
- mod_muc: new hooks
- mod_push: Support XEP-0357: Push Notifications
- mod_push_keepalive: New module

PubSub/PEP
- Keep disco#info on PEP compatible with XEP-0060
- Preliminary export PubSub data from Mnesia tables to SQL file
- Fix PubSub send last published items
- Fix PEP node removal
- Fix PEP node identity
- Fix disco#items on PEP service
- Fix getting cached last item
- Add import of PEP from prosody

(fhajny)