Add a new abstraction called span store in bor consensus. This makes the header verification stateless as instead of fetching validator set (of a span) from genesis contract, we rely on span from heimdall itself for validator set. This doesn't change any assumptions for consensus as the data in genesis contracts is written by fetching span details from heimdall in the first place.
This helps us with multiple things such as header verification for snap sync, validating producer set against header's extra data, and using heimdall as a source of truth instead of making a contract call to genesis contract which generally leads to issues when importing side chains (as we need to use the side chain state and not canonical).
Span store is a minimal cache which maintains last 10 spans in memory. It provide simple read methods via span id and block number so that we can use it in consensus.
