Stress test for memory-based admission control

We should extend the existing stress test to have a new mode designed to test memory-based admission control, where the stress test framework does not try to throttle memory consumption but instead relies on Impala doing so.

The required changes would be:

• A mode to disable throttling
• Options for stricter pass conditions - queries should not fail with OOM even if the stress test tries to submit way too many queries.
• However AC queue timeouts may be ok.
• Investigation into the logic for choosing which query to run next and when - does that need to change?