Introducing eval-service and porting scotty-pastebin to new Worker paradigm

- Moving some stuff around, particularly see 'Eval.Worker.Internal'

- Introducing 'eval-service' that listens of control socket and controls
  the workers pool

- Making sure that pastebin works with eval-service

- Some examples in the 'examples/' folder

- Minor tweaks
  - EvalCmd, ServiceCmd, LimitSettings can be serialized
  - Build scotty-pastebin with threaded