Feat: Fetch missing blocks

[0xKitsune]

This PR introduces logic to fetch missing blocks due to any potential interruptions of the index_chain task. Rather than spinning up a new task, this approach backfills blocks before listening to the blocks_stream. Since the WsClient's subscribe_blocks implementation in ethers-rs uses an UnboundedReciever, new blocks are buffered as backfilling occurs.

pub async fn index_chain(app: Arc<App>, chain_id: u64) -> eyre::Result<()> {
    loop {
        let ws_rpc = app.ws_provider(chain_id).await?;
        let rpc = app.http_provider(chain_id).await?;

        // Subscribe to new block with the WS client which uses an unbounded receiver, buffering the stream
        let mut blocks_stream = ws_rpc.subscribe_blocks().await?;

        // Get the first block from the stream, backfilling any missing blocks from the latest block in the db to the chain head
        if let Some(latest_block) = blocks_stream.next().await {
            backfill_to_block(app.clone(), chain_id, &rpc, latest_block)
                .await?;
        }

        // Index incoming blocks from the stream
        while let Some(block) = blocks_stream.next().await {
            index_block(app.clone(), chain_id, &rpc, block).await?;
        }
    }
}