deduplicate logic
create more producer goroutines (torrent lib does limiting internally
amount of consumers/disk-readers/hashers by 2, and it's enough because
we can verify multiple files in parallel)
move flag from "downloader torrent_hashes --verify" to "downloader
--verify"
On power-off recent "no-sync" txs may be lost. It will cause 2. cases:
- Good piece on disk and recent "complete" db marker lost. Self-Heal by
re-download.
- Bad piece on dis and recent "incomplete" db marker lost. No Self-Heal.
So, can't afford loosing recent "incomplete" markers.
