Cuts As Cancellation Tokens

[G3Kappa]

1. Create a CancellationTokenSource when Solve is called
2. Attach its token to the SolverScope
3. Check whether the cancellation for that token was requested instead of using a volatile bool

[G3Kappa]

Changes

• Implemented SolverContext to encapsulate appropriately the SolverScope and the ExceptionCancellationTokenSource during the calls to Solve().
• Implemented cuts as a CancellationToken that's tied to a CancellationTokenSource hidden within the SolverScope. Choice points are introduced naturally by modifying the scope in place to "forget" about the previous cancellation source, and by encapsulating dependent queries in a new solver context.