Improved microbenchmarking with multiple features.

* inline performance critical code
* Average runtime is specified and used to calculate iterations.
* Console: show median of multiple runs
* plot: show box plot
* filter benchmarks
* specify scaling factor
* ignore src/test and src/bench in command line check script
* number of iterations instead of time
* Replaced runtime in BENCHMARK makro number of iterations.
* Added -? to bench_bitcoin
* Benchmark plotly.js URL, width, height can be customized
* Fixed incorrect precision warning

src/bench/prevector_destructor.cpp

@@ -32,5 +32,5 @@ static void PrevectorClear(benchmark::State& state)
     }
 }
 
-BENCHMARK(PrevectorDestructor);
-BENCHMARK(PrevectorClear);
+BENCHMARK(PrevectorDestructor, 5700);
+BENCHMARK(PrevectorClear, 5600);