add the possibility to execute the curl in multiple threads in the redis-rest-storage-performance.rb

[floriankammermann]

In the moment we execute the curl from one thread. This is enough to get the execution time of a rest-storage call when only one client is active.

To gain the execution time under load, we should add the possibility to distribute the curl commands over multiple threads.

The solution should be, that we have an additional parameter "-t" where we can define the number of threads which should be used to execute the curl.

[mcweba]

See http://t-a-w.blogspot.ch/2010/05/very-simple-parallelization-with-ruby.html