Refactor handling of interruptions

src/main/scala/leon/LeonContext.scala

src/main/scala/leon/Main.scala

src/main/scala/leon/Reporter.scala

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

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

src/main/scala/leon/synthesis/LinearEquations.scala

src/main/scala/leon/synthesis/ParallelSearch.scala

src/main/scala/leon/synthesis/SearchCostModel.scala

src/main/scala/leon/synthesis/SimpleSearch.scala

src/main/scala/leon/synthesis/SynthesisContext.scala

src/main/scala/leon/synthesis/Synthesizer.scala

src/main/scala/leon/synthesis/TaskRunRule.scala

src/main/scala/leon/synthesis/TaskTryRules.scala

src/main/scala/leon/synthesis/rules/Cegis.scala

src/main/scala/leon/synthesis/rules/IntegerEquation.scala

src/main/scala/leon/synthesis/utils/Benchmarks.scala

src/main/scala/leon/utils/InterruptManager.scala

src/main/scala/leon/utils/Interruptible.scala

src/main/scala/leon/utils/Stopwatch.scala

src/main/scala/leon/utils/SubtypingPhase.scala