class type workload_manager = object .. end
method hello : controller -> unit
Called by the controller when the service is added
method shutdown : unit -> unit
Called by the controller to notify the manager about a shutdown
method adjust : socket_service -> socket_controller -> unit
This function is called by the controller at certain events to
 adjust the number of available containers. The manager can
 call 
start_containers and 
stop_containers to change the
 system.
 The function is called right after the startup to ensure
 that there are containers to serve requests. It is also called:
- just after a connection has been accepted and before it is
   decided which container will have the chance to accept in the
   round
- after the shutdown of a container
 Of course, the workload manager is free to adjust the load
 at any other time, too, not only when 
adjust is called.
method capacity : container_id ->
       container_state -> capacity
Computes the capacity, i.e. the number of jobs a certain container
 can accept in addition to the existing load.