Replace four time.sleep(0.01) timestamp guards in
memory_service_coverage_steps.py with a deterministic
_wait_for_clock_advance() helper that busy-waits on the monotonic
clock until the UTC clock actually advances past the recorded
before timestamp, bounded by a 2-second deadline.
Also introduce memory_service_clock_wait.feature and its step
definitions to verify the helper raises AssertionError on deadline
exceeded and returns normally once the clock advances.
Closes#9963
