[Users] dispatcher has failover support

[Daniel-Constantin Mierla]

Hello,

the dispatcher module (kind of fast load balancing support) has now 
support for failover. Along with selected destination, the rest of 
addresses in the destination set are stored in AVP list. If the selected 
destination fails to manage the request, then the next address can be 
used from failure_route.

Some destinations can be dynamically marked as inactive (e.g., based on 
reply code) so the are not going to be used until the admin enables them 
again, via fifo command.

For more info, please see the readme file:
http://openser.org/docs/modules/1.1.x/dispatcher.html

Cheers,
Daniel