Add support for file-based queues

Add support for file-based queues

It's now possible to use files as internal packet queues.
The following options are introduced:

* queue_type: the option can be set to `ram` (default) or `file`.
  The option can be set per virtual host.
* queue_dir: path to the directory where queues will be allocated.
  The default is 'queue' directory inside Mnesia directory.
  This is a global option and cannot be set per virtual host.