Use 'infinity' instead of 'unlimited'

Use `infinity` for consistency with other ejabberd settings.  This just updates a record definition; the actual code should already cope with `infinity`: The `max_allocs` field currently [ends up][1] being unused; and any atom
should work for `max_permissions`, as it [checks][2] `Integer < atom`, and that's true for any atom.

[1]: https://github.com/processone/stun/blob/89dd097075fcc7d6ee00b218c946a3ea9d092f8b/src/turn_sm.erl#L62
[2]: https://github.com/processone/stun/blob/89dd097075fcc7d6ee00b218c946a3ea9d092f8b/src/turn.erl#L215