opt arguments support http protocol (default use HTTP/1.1)

1. add protocol arg.
2. fix gcc make warning in wrk.c 
src/wrk.c:139:56: warning: format specifies type 'size_t' (aka 'unsigned long') but the argument has type
      'uint64_t' (aka 'unsigned long long') [-Wformat]
            fprintf(stderr, "unable to create thread %zu %s\n", i, msg);