Account based MUC joining

=========================

Normally when joining a MUC groupchat, it is each individual client that

joins. This means their presence in the group is tied to the session,

which can be short-lived or unstable, especially in the case of mobile

clients.

This has a few problems. For one, for every message to the groupchat, a

copy is sent to each joined client. This means that at the account

level, each message would pass by once for each client that is joined,

making it difficult to archive these messages in the users personal

archive.

A potentially better approach would be that the user account itself is

the entity that joins the groupchat. Since the account is an entity that

lives in the server itself, and the server tends to be online on a good

connection most of the time, this may improve the experience and

simplify some problems.

This is one of the essential changes in the MIX architecture, which is

being designed to replace MUC.

`mod_minimix` is an experiment meant to determine if things can be

improved without replacing the entire MUC standard. It works by

pretending to each client that nothing is different and that they are

joining MUCs directly, but behind the scenes, it arranges it such that

only the account itself joins each groupchat. Which sessions have joined

which groups are kept track of. Groupchat messages are then forked to

those sessions, similar to how normal chat messages work.

Known issues

------------

-   You can never leave.

-   You will never see anyone leave.

Unknown issues

--------------

-   Probably many.

Compatibility

=============

Briefly tested with Prosody trunk (as of this writing).