feat: write output for manifest generation in parallel

[luizirber]

While the current implementation is simple, it is also

• serial, reading one sig at a time
• only write output at the end, when all sigs are processed

The changes here allow reading sigs in parallel, and writing manifest records as they are generated, which can be useful when building manifests for huge collections (like wort), and restarting calculation without recalculating everything.