remove DeltaVision-specific constant from pixels service
create new disk usage request
implement new disk usage request
make new request callable

(add thumbnail service into request object factory registry)
adjust API of disk usage request
adjust disk usage implementation to match API change
extend disk usage object with file counts
implement file counts for disk usage requests

refactors usage tracking to fit extra duties
Merge remote-tracking branch 'remotes/origin/develop' into trac-7240-disk-usage-count
Merge pull request #2919 from mtbc/trac-7240-disk-usage-count

fix #7240: provide disk usage count