Optimize sequencing procedure

[Troublor]

Currently sequencer needs to preparePayloadAttributes before start building the next L2 block.

optimism/op-node/rollup/sequencing/sequencer.go

Line 495 in ec45f66

attrs, err := d.attrBuilder.PreparePayloadAttributes(fetchCtx, l2Head, l1Origin.ID())

However, preparePayloadAttributes functions rely on eth_getBlockByHash JSON-RPC to fetch the L2 system config by obtaining the sequenced L2 block:

optimism/op-service/sources/l2_client.go

Line 161 in 9480fcd

envelope, err := s.PayloadByHash(ctx, hash)

However, L2 block may be too large so that eth_getBlockByHash JSON-RPC takes a long time, blocking the process of the sequencer.
In fact, only the first transaction in the previous L2 block is needed to fetch the L2 system config.
Therefore, this PR avoids eth_getBlockByHash JSON-RPC and use eth_getTransactionByBlockHashAndIndex to fetch only the first transaction in the previous L2 block.
Thus reducing latency in sequencing process.

This PR relies on https://github.com/megaeth-labs/mega-reth/pull/115

[claymega]

LGTM