Make disk usage monitoring work for the index caching feature (PR #612)

[tlwillke]

The existing disk usage monitoring does not work with the index caching feature introduced in #612. This is because the monitor monitors only the temp working directory in which ephemeral indices are built. It also uses a delta approach to measuring footprint and file count that isn't compatible with reporting the size and file count of a persistent (cached) index.

We need to see if we can extend disk usage monitoring to capture both ephemeral and persistent index footprints. It would also be helpful if the footprint was logged / output as an index construction property instead of a query property in the benchmark tests.