FS-9079: [mod_callcenter] Add ring-progressively strategy

A new strategy is now available through mod_callcenter, which aim
is to provide a mix between top-down and ring-all strategies.

If set to ring-progressively, the strategy will call an agent, and
then a second agent, and then a third agent, without cancelling
previous calls.

The delay between each call is 10 sec by default, but it can be
changed using the queue variable ring-progressively-delay.