During start of application we start several(4) actors of MigrationActor type.
Routing of messages to these actors managed by BalancingPool
Scheduler retrieve data items using DataProvider (it could be usual dao instead)
and send them one by one to MigrationActors
After when actor received message he performs call to ExternalService.
This call takes some time - we perform delay on ExternalService side.
It allows to show how to Akka works with multithreading
To get count of actors use GET operation on http://localhost:8123/actorsCount
To change count of actors use POST operation on http://localhost:8123/actorsCount/{change}, where {change} could be positive or negative