Before, when solving a solver, parsing sub objects of solver_process was done directly synchronously
in solver_workflow. And we had the case where you have parallel transactions solving a solver,
each of theses transactions see remaining solver not in solved state, but once all transaction are
finished, all solver are solved. This could lead to the case where solver_process is never moved
to succeeded. Instead of using serialize (which may lead to conflicts), just use activities in queue
with a serialization tag.