add facility to dispense with cancellation (not activated at this point). Address #961 by expanding recurisve function definitions that are not tautologies if the current model does not validate

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/sat/sat_solver.cpp

@@ -33,6 +33,7 @@ namespace sat {
 
     solver::solver(params_ref const & p, reslimit& l, extension * ext):
         m_rlimit(l),
+        m_checkpoint_enabled(true),
         m_config(p),
         m_ext(ext),
         m_par(0),