SolverManager Race Condition: Double Terminate when SOLVING_SCHEDULED

If `terminateEarly` is called from two separate threads when SOLVING_SCHEDULED, it is possible for two threads to be in solvingTerminated at once, which allows this sequence of events:

1. Thread 1, Thread 2 are in solvingTerminated
2. Thread 1 unregister the solver job, which allows a new solver job to be created with the same Problem Id
3. Thread 3 creates a new SolverJob with the same Problem Id
4. Thread 2 unregister the solver job

Now Thread 3 have a SolverJob with a given problemId (that is either SOLVING_SCHEDULED or SOLVING), yet solverManager.getSolverStatus(problemId) == NOT_SOLVING. This allows for another SolverJob to be created with the same problemId, that can be cancelled via the SolverJob Thread 3 has.

The race condition is extremely hard to trigger.