add some multithreading idea to benchmark.
add multithreading idea to example/benchmark.
add stopListening() to QHttpServer.
fix multithreading bugs in example/benchmark.
client's params are set before moveToThread().
add USETHREAD config in example/benchmark project.

now building benchmark as a multithread application is optional.
put thread declaration of benchmark classes.
add command line argument for multithreading in benchmark app.

the single-thread / multi-thread working mode of the app is set by
command line argument.
update README.md for new multi-threading benchmark.

also benchmark/README.md has been updated.