[WIP, POC] Unified threading model in MIP solver

[nguidotti]

This PR unifies the threading model across the MIP solver, such that the code now uses the tasking model from OpenMP. The only exception is the Papilo presolver that uses Intel TBB.

More specifically, this PR

• Solves the CPU oversubscription problem. The solver now respect the number of threads set to the user, with the exception of Papilo or threads created by the CUDA runtime.
• Removes overheads from creating and destroying std::thread. This also
• Migrates RINS from std::thread to omp task. Similar to previous logic, one instance of RINS can run at a time.
• Migrates CPU FJ from std::thread to omp task. There are a few limitations
  • scratch_cpu_fj_on_lp_opt and scratch_cpu_fj are running for the entire program. This essentially allocate two dedicated threads to these functions, while other routines needs to share the remaining CPU resources. This may hurt the performance for low core count CPUs.
  • Since there is a small delay between the task creation and its start (since the threads may be busy), the GPU FJ may finish before the CPU FJ even start when racing.
• Replaces locks in the pseudocost with atomics to reduce contention.

Checklist

• I am familiar with the Contributing Guidelines.
• Testing
  • New or existing tests cover these changes
  • Added tests
  • Created an issue to follow-up
  • NA
• Documentation
  • The documentation is up to date with these changes
  • Added new documentation
  • NA

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