* Accessing the internals of the SAT solver

The non-public parts of the SAT solver are accessed via the static methods in 
the SatSolverProxy class. SatSolverProxy is declared as a friend of the 
SatSolver and has all-privileges access to the internals -- use with care!!!

* Clause Database and CNF

The clause database consists of two parts:

    vec<Clause*>        clauses;          // List of problem clauses.
    vec<Clause*>        learnts;          // List of learnt clauses.

Clauses is the original problem clauses, and learnts are the clauses learned 
during the search. I have disabled removal of satisfied problem clauses by 
setting the remove_satisfied flag to false.

The learnt clauses get removed every once in a while by removing the half of
clauses with the low activity (reduceDB())

Since the clause database backtracks with the SMT solver, the CNF cache should
be context dependent and everything will be in sync. 

* Adding a Clause

The only method in the interface that allows addition of clauses in MiniSAT is

    bool Solver::addClause(vec<Lit>& ps),

but it only adds the problem clauses. 

In order to add the clauses to the removable database the interface is now 

    bool Solver::addClause(vec<Lit>& ps, bool removable).      

Clauses added with removable=true might get removed by the SAT solver when 
compacting the database.

The question is whether to add the propagation/conflict lemmas as removable or 
not?

* Making it Backtrackable

First, whenever we push a context, we have to know which clauses to remove from
the clauses vector (the problem clauses). For this we keep a CDO<int> that tells
us how many clauses are in the database. 

We do the same for the learnt (removable) clauses, but this involves a little 
bit more work. When removing clauses, minisat will sort the learnt clauses and 
then remove the first half on non-locked clauses. We remember a CDO<int> for 
the current context and sort/remove from that index on in the vector. 

Also, each time we push a context, we need to remember the SAT solvers decision 
level in order to make it the "0" decision level. We also keep this in a 
CDO<int>, but the actual level has to be kept in the SAT solver and hard-coded 
in the routines.

* Literal Activities

We do not backtrack literal activities. This does not semantically change the 
equivalence of the context, but does change solve times if the same problem is
run several times.

* Conflict Analysis

TODO

* Do we need to assign literals that only appear in satisfied clauses?