Add unverified blocks root

[ata-nas]

Instead of storing block files directly in the local destination, we wish to modify the block writers as follows:

1. Store block files initially in an "unverified" folder, just flat with full file name.
2. A separate process initiated when a result is published by the verification service will move the file.
   • When a "verified" result is published this will move the now-verified block (only) to its proper destination
   • When an "unverified" result is published, this will remove the now-failed block (only).
   • When other result types are published this sub-service will only react if appropriate.
3. On startup, the persistence service will remove all unverified files.
   • This might be accomplished by using a folder on tmpfs, which is managed in-memory by the kernel and would significantly improve reliability and performance.
4. All other persistence service elements, and all archive services, will operate only on the trie structure folders containing verified block files.