Use ThreadLoca<DateFormat> in SimpleLogger

In ```SimpleLogger``` is used static ```DateFormatter``` guarded by itself. I thought it might be good to replace it with ```ThreadLocal<DateFormatter>```. I think it's faster by ~3-5%, but would be good if someone other can verify the results.

    With LocalThread
    Benchmark                                      Mode  Samples       Score      Error  Units
    o.s.i.SimpleLoggerBenchmark.eightThreads      thrpt       20  291713.679 ± 2090.442  ops/s
    o.s.i.SimpleLoggerBenchmark.fourThreads       thrpt       20  291552.450 ± 1771.590  ops/s
    o.s.i.SimpleLoggerBenchmark.oneThread         thrpt       20  548539.921 ± 3197.140  ops/s
    o.s.i.SimpleLoggerBenchmark.sixteenThreads    thrpt       20  281860.846 ± 3172.071  ops/s
    o.s.i.SimpleLoggerBenchmark.twoThreads        thrpt       20  290422.132 ± 2213.392  ops/s
    
    With synchronized block
    Benchmark                                      Mode  Samples       Score       Error  Units
    o.s.i.SimpleLoggerBenchmark.eightThreads      thrpt       20  251927.856 ±  4606.081  ops/s
    o.s.i.SimpleLoggerBenchmark.fourThreads       thrpt       20  289278.497 ±  1899.424  ops/s
    o.s.i.SimpleLoggerBenchmark.oneThread         thrpt       20  550569.540 ±  4470.577  ops/s
    o.s.i.SimpleLoggerBenchmark.sixteenThreads    thrpt       20  275318.387 ±  3156.346  ops/s
    o.s.i.SimpleLoggerBenchmark.twoThreads        thrpt       20  295816.125 ± 24218.907  ops/s

There are new unit tests using jmockit and jmh benchmarks in new module ```slf4j-perf```