Run multiple instances of ProB

[bivab]

The idea is to have several instances of ProB with the same model running in the background.

The number of instances should depend on the system memory (n relation to how much memory we consume per instance).

• As soon as the first instance is ready we can start accepting user requests.
• Move Tasks should be dispatched to all executors, to ensure we have a consistent state across all instances.
• All other tasks should be dispatched to any free ProB executor.

[bivab]

Options so far seem to be:

• Run and manage multiple instances on top of ProB 2.0, manual synchronization when state modifications happen.
  • Disadvantage slow startup when having to parse and compute constants in each instance.
• Save mutable state (.sav) in prob-prolog and use it to start additional instances
  • If this is cheap, all but one instances can be discarded and restarted upon state modification.
• Forking is not really an alternative as it is too brittle and does not work on windows.