Add a flag to allow using multiple solvers at the same time

[yav]

Different solvers perform better on different problems. To gain the best of both worlds, we may want to issue a problem to more than one solver at the same time.

Potentials challenges:

• We need to investigate if there is a way to interrupt a solver that is taking a long time.
• Worst case we can just kill the process, but we need to be able to rebuild the solver's state, to get it ready for the next query.

[cp526]

Setting up CN so it always runs both cvc5 and z3 in parallel would be really good, and might noticeably improve CN performance for practical purpose.

Here's a recent issue where the two solvers result in drastically different performance: #613 .