v1.18: blockstore: always send a coding shred for chained merkle root conflicts (backport of #1353)

[mergify]

Problem

#835 has a chance to send a chained merkle root conflict duplicate proof consisting of 2 data shreds.
With 2 data shreds we cannot verify that they are from adjacent FEC sets. We require that the shred from the lower FEC set be a coding shred.

Summary of Changes

• Modify the backwards check to always send the coding shred from lower FEC set. We know this is available because there must be an erasure meta.
• Modify tests to check the upgrade scenario, where the first received coding shred index is not reliable (skip the check in this case).

This ensures that we always send a coding shred for the lower FEC set, meaning the gossip handler can verify that duplicate proofs are for adjacent FEC sets.
Without this verification an attacker can arbitrarily pick non adjacent FEC sets to "prove" that every block is duplicate

Contributes to solana-labs#34897

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This is an automatic backport of pull request #1353 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   ledger/src/blockstore_meta.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   ledger/src/blockstore.rs

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[AshwinSekar]

This backport fixes a bug in the previous change #1009. The previous change allowed a duplicate proof to be created with a data shred, which is unverifiable through gossip. This prevents the feature flag for chained merkle root duplicate proofs from being turned on.

With this change, we modify the proof creation to always use a coding shred. We ignore and log if any errors are encountered, and there are tests verifying that upgrading from an older blockstore does not cause any panics.