Coordinator to kick off a new run when loading queues are sufficiently empty

After #7159 is fixed, Coordinator can start a new run as soon as the total number of loading segments drops below the threshold, instead of waiting the full period (default is 1 minute). In some cases, it may make Coordinator be idle almost two times less (60.1 seconds vs. 120 seconds).

Start by reviewing issue #7159 and the Coordinator's loading-queue and run-wait behavior. The change is complete when Coordinator can begin a new run once total loading segments fall below the threshold, without waiting for the full default period, and the behavior is covered by the relevant tests.

Written by the indexing model from the issue text.