Trying to connect to a remote instance using direct s2s won't work if local instance has not the feature enabled, and if the remote instance does not know the local one.
So using Websocket S2S in such case (that embed a discovery mecanism).
The data format used by plugin v6.3.0 was not well suited.
Here comes a new data format, with S2S informations.
The plugin can automatically upgrade old format.
It also continues to provide the old format, so than remote instance
that did not update the plugin will still work.
Note: websocket s2s is not working yet, still WIP.
New Features
* Chat Federation:
* You can now connect to a remote chat with your local account.
* This remote connection is done using a custom implementation of [XEP-0468: WebSocket S2S](https://xmpp.org/extensions/xep-0468.html), using some specific discovering method (so that it will work without any DNS configuration).
Minor changes and fixes
* Possibility to debug Prosody in development environments.
* Using process.spawn instead of process.exec to launch Prosody (safer, and more optimal).
* Prosody AppImage: fix path mapping: we only map necessary /etc/ subdir, so that the AppImage can access to /etc/resolv.conf, /etc/hosts, ...
* Prosody AppImage: hidden debug mode to disable lua-unbound, that seems broken in some docker dev environments.
* if both local and remote instance have external XMPP connections enabled, the user joins the remote room with his local account
* some code refactoring (builtin.ts)
Note: documentation and settings descriptions are to do.
Related to #112
