Make Thor slaves dali clients via configuration option

Currently, thor slaves are always dali clients.
There's very little in a thor slave that requires dali connectivity - the only exception I can think of is if a plugin tries to connect to dali, e.g. fileservices.

This in itself has caused grief in the past, causing a type of DoS attack on Dali, as slaves flood dali with what is often an unintentional or at least unwise construct.

Add a Thor configuration option to disable the connectivity and make it defaulted to off - hopefully there are few if any relying on this.

One of the motivation is that, with so many dali clients, the Dali select handler I think can become overwhelmed and start to burn a lot of cpu cycles just monitoring all the connections. This may be in turn dragging down the performance of Dali in environments with 1000's of clients.