Migrate the existing PBFT consensus to a Rollup-style sequencer architecture

[FletcherMan]

Background

This is the first phase for #846, migrating PBFT consensus to a centralized sequencer, while maintaining compatibility with both the legacy and the new block production logic.

Reference

https://larkfive.sg.larksuite.com/wiki/EE2FwFjmzi7mNlkTkVJlguXFgUh