This ensures that the same solver_types module is not reused
for two different solvers, which would be problematic.
Marked minor because of the low use of multiple instances of a solver.
When the solver finds a SAT result, it sends the whole
model to the theory, because maybe it can do something
interesting/costly to expand the proof search. After
that there must be a check to see if the theory has effectively done
something, in which case we should resume proof search, or if nothing
has been done, in which case the solver should return that the problem
is satisfiable. That check was incorrect before (checking number of
assumptions, and if the queue is all caught up), because new learnt
clauses (i.e tautologies, which are *not* assumptions) can be added that
do not immediately causes propagation, so that the number of assumptions
and the element queue is constant, but we should still resume the
search.
