Add simulation test to ensure each storage server has similar space utilization

We want each storage server has similar space utilization for balancing the disk space pressure among storage servers. For example, if each storage server has the same amount of space, we want each storage server has similar amount (e.g., +-5% difference) of data.

The current implementation only test that there is no data movement when a cluster reaches to a stable state. However, it is possible that the cluster only uses a set of storage servers and balance the data on them, leaving some storage servers empty.

We can add the test in `waitForQuietDatabase()` to check the shards size on each storage server, and make sure the standard deviation is not too big (say within +-5% of the average size).

Start by reading waitForQuietDatabase() and the existing simulation test that checks for no data movement after the cluster stabilizes. Inspect how shard sizes are obtained for each storage server, then add the requested utilization check and verify that the test rejects distributions outside the stated +-5% average range.

Written by the indexing model from the issue text.