Write new stress test using Python-based testing framework.

Some work was already done on rewriting the existing system tests in Python (see #1062), but this has not been completed yet.

The stress test is becoming more and more painful to maintain and extend, especially w.r.t. testing the Postgres scheduler.

This issue is to keep track of the work to write the stress test in Python using the test framework developed for #1062. This allows us to build some basic ingredients, which we can eventually use to migrate the rest of the system tests.

The plan of attack is to first get a very simple test running using the Python framework. Then we can build the basic functions to do archival, retrieval, eviction etc. These functions should be flexible in how they are executed (e.g. number of files, size of the files, prefix name of the file). Once we have these basic ingredients, setting up the stress test should be a simple matter of chaining a few of these calls together (and put some big numbers in there).