Bound parent block fetches from unknown-parent data column sidecars

[terencechain]

• An unknown parent data column sidecar spawned an uncapped goroutine calling sendBatchRootRequest, so a peer could use junk parent root per sidecar request peers with beacon_blocks_by_root
• Dedupes roots already in flight and caps distinct fetches per slot, and uses s.ctx instead of context.Background() so shutdown cancels

[daniel5713]

Great!