MINOR: proxy: assign dynamic proxy ID

MINOR: proxy: assign dynamic proxy ID

Implement proxy ID generation for dynamic backends. This is performed
through the already function existing proxy_get_next_id().

As an optimization, lookup will performed starting from a global
variable <dynpx_next_id>. It is initialized to the greatest ID assigned
after parsing, and updated each time a backend instance is created. When
backend deletion will be implemented, it could be lowered to the newly
available slot.