Split up SolverConfig and SolverFactory

For quarkus and spring-boot, it should be possible to load a solver config xml, modify it and then build a SolverFactory from it.
Even more, for Quarkus, we need to do this all at compile time and save the SolverFactory element in memory.

Furthermore, we can do additional caching to improve performance when building 10 000 solvers from the same solverfactory.

(Users shouldn't have to deal with SolverConfig unless they want to - the current ways to build a SolverFactory will remain too.)