Add asynchronous incremental prefetch refresh

[RenanGBarreto]

Why

Metadata and selected file regions may need refresh while device remains active. Refresh must never block USB activation or mutate an active generation.

Plan

• Start cancellable workers for immutable generations.
• Run refresh asynchronously at a configurable 300-second interval.
• Read through existing NBD/mapper path so nbdkit remains sole verified file-cache layer.
• Track covered ranges and refresh only missing or changed blocks where reliable source identity exists.
• Stop and join workers during reload, eject, shutdown, and generation replacement.
• Bound subprocess work, CPU, and concurrent refresh operations.

Acceptance criteria

• Activation does not wait for optional refresh.
• Refresh failures do not tear down a healthy active generation.
• Workers stop promptly and leave no subprocess behind.
• Tests cover cancellation, replacement, failure, and interval behavior.