fix(dragonfly_test): failing bug207 on build opt

[kostasrim]

The issue was that the Heartbeat would run every 1ms and this is problematic because the for-loop would take less than 1ms to finish. Therefore, the memory pool would not adjust and items would not be evicted from the store. By doubling the amount of elements created in the for-loop, we give enough time for the first heartbeat to run and adjust the memory available (which will cause the evictions to happen).

This passes locally but if we see flakes on the CI we can slightly increase the elements we create in the forloop again.

[kostasrim]

Do not review, I will make another change and make RunPeriodic use nanoseconds instead of milliseconds. a) They get converted anyway to nanoseconds since helio uses steady_clock::duration which uses nanoseconds. b) it's a better solution

Small update @romange, since your suggestion fixes the issue I am fine for now with this fix (That is 10k calls to SETEX instead of 5k). as my suggestion above needs to do changes in helio and I don't want to delay this PR (since our tests are broken atm)

[romange]

dcheck at db_slice.cc:155 is incorrect. should be DCHECK_LE