v3.1: blockstore: Add fallback in mark_slot_dead_if_not_full() (backport of #12026)

[mergify]

Problem

This method currently only checks the ShredInsertionTracker for a slot meta.

Summary of Changes

Fallback to the blockstore as an extra check if we don't find a meta in the tracker + add unit test

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This is an automatic backport of pull request #12026 done by Mergify.

[mergify]

Cherry-pick of 9808d65 has failed:

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	both modified:   ledger/src/blockstore.rs

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