Bettern handling of incremental solvers

src/main/scala/leon/solvers/IncrementalSolver.scala

src/main/scala/leon/solvers/ParallelSolver.scala

src/main/scala/leon/solvers/RandomSolver.scala

src/main/scala/leon/solvers/Solver.scala

src/main/scala/leon/solvers/TimeoutSolver.scala

src/main/scala/leon/solvers/TrivialSolver.scala

src/main/scala/leon/solvers/z3/AbstractZ3Solver.scala

src/main/scala/leon/solvers/z3/FairZ3Solver.scala

src/main/scala/leon/solvers/z3/UninterpretedZ3Solver.scala