PaB: keep former baker one more block

When the protocol agnostic baker switch to a new baker (for a new protocol Y) it shutdowns the previous one (protocol X).

This is an issue as the last block of a protocol X may be a high round, and thus the former baker X is still needed to allow the chain to move on. Indeed, while the activation block is not finalized, we potentially need to use baker of protocol X. IIUC, we need to keep baker X only one more block.