Some fixes for stress test script

Stress test from the master branch doesn't work. It uses URLs that are not used in CMS, probably these URLs are from previous version. I fixed it for my purpose, and, hope, it will be useful for somebody else too.

Also I've added a couple features like:
- output the time it took request to complete in console log (I really missed it during testing)
- allow to run StressTest.py from outside of the local network, without direct connection database. The database connection only needed for "harvesting" contest data. So I've added two options that allow to "prepare" contest data (i.e., export essential users and task information into a file), and "reading" contest data from the file after that.