dl: missed Del follow-up

- agg: to send correct list of created files. now it's empty `files changed...sending notification l=[]`
- agg: SaticFiles - to hold `*dirtyFilesItem` instead of same fields. Then can use `.Paths(rel)` method
- higher level unit-test - to see what is in DL
- pruneBlockSnapshots: push-down this logic into `blockRetire`: instead of work on raw file names - we must work on dirtySegments. Instead of calling downloader.Del(file) we must call `downloader.Del(dirtySegment.Paths(snapDir)`
- `RoSnapshots) RemoveOverlaps` may remove Caplin's .tmp files
- `Merger.Merge`: //TODO: or move it inside `integrateMergedDirtyFiles`, or move `integrateMergedDirtyFiles` here. Merge can be long - means call `integrateMergedDirtyFiles` earliear can make sense.