Sync state of folders

[jkawamoto]

The state of each folder is determined by integrating sync states of files in the folder. Currently, to integrate those states, OverlayHelper walks the file tree. However, since siad downloads files in some place first, there are no files in the file tree until the download is finished. Hence, OverlayHelper decides the sync state of the folder "idle" even if some files are being downloaded.

Instead of walking the file tree, it seems better to query the sync DB so that OverlayHelper can find such downloading file.