Investigate sporadic bursts in CPU and memory usage during batch cancel test

[randomir]

Running the tests.test_solver.Submission.test_cancel_batch test, sometimes causes CPU and/or memory usage to increase to 100%/3-4GiB during prolonged test time of few minutes. Usually the test runs within 1m, but it has been observed to take 10m.

It is possible the reason for this is SAPI/QPU load, i.e. user throttling. Even if the reason is SAPI user throttling, the CPU/memory impact should be curbed.

[randomir]

When submitting 10 jobs to a 2000Q, runtime is ~120 s and peak memory usage ~800 MiB. When submitting 100 jobs to a 2000Q, runtime is ~13.5 min and peak memory usage ~4.7 GiB.

When submitting 10 jobs to a C4 software solver, runtime is ~7 s and peak memory usage ~70 MiB. When submitting 100 jobs to a C4 software solver, runtime is ~15 s and peak memory usage ~100 MiB.

Cancel rate is always around 65%