tsan documentation says 2-20x, so set it at 20x
asan documentation says 1.2-2.7x, so set it at 3x
msan documentation says 2-4x, so set it at 4x
This is now much less optimistic than before

2. Reactive tsan tests for qps_test
3. Set CPU load for qps_openloop_test
4. Divide qps_openloop_test Poisson rate by the slowdown factor of
   the configuration

Importantly:
- allows matching timeout values between makefile and run_tests without
  repeating ourselves
- allows borrowing of compiler flags by other build systems

There's still a little too much build configuration built into our
Makefile, but we can start attacking that over time.