apollo_state_sync: fix bug in state sync get_block_hash

[ShahakShama]

No description provided.

[reviewable-StarkWare]

This change is 

[Itay-Tsabary-Starkware]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion

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crates/apollo_state_sync/src/lib.rs line 172 at r2 (raw file):

                // As a fallback, try to get the block hash through the feeder directly. This
                // method is faster than get_block which the sync runner uses.
                // TODO(shahak): Test this flow.

What about this?

Code quote:

 // TODO(shahak): Test this flow.

[Itay-Tsabary-Starkware]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @ShahakShama)

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crates/apollo_state_sync/src/lib.rs line 166 at r2 (raw file):

            )
        })
        .await??;

Please add comment explaining what's happening here.
Also, why do we need a spawn_blocking here?

Code quote:

        let block_hash_opt = tokio::task::spawn_blocking(move || {
            Ok::<_, StateSyncError>(
                storage_reader
                    .begin_ro_txn()?
                    .get_block_header(block_number)?
                    .map(|header| header.block_hash),
            )
        })
        .await??;