Bug 6015, new script for load testing

Bug 6015, new script for load testing

This benchmark_staff.pl script is based on the previous benchmark_circulation.pl script
As it does not test only circulation, the renaming was necessary.
The script has many enhancement compared to the benchmark_circulation.pl one
The benchmark_staff will run a benchmark on the following pages:
    * mainpage.pl
    * catalogue/detail.pl
    * catalogue/search.pl
    * members/member.pl (search on a name)
    * members/member.pl (search on a 1st letter)
    * circulation/circulation.pl and return.pl (check-out and check-in)
    * all those steps at the same time

    run the script without any parameter to get syntax