Improve timers collection, usage and display

src/main/scala/leon/LeonContext.scala

src/main/scala/leon/Main.scala

src/main/scala/leon/frontends/scalac/ExtractionPhase.scala

src/main/scala/leon/solvers/EnumerationSolver.scala

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

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

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

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

...cala/leon/synthesis/search/AndOrGraphParallelSearch.scala

src/main/scala/leon/utils/Timer.scala