Discard incompatible baker stored states

Context

This MR prevents the baker from crashing when a stored baker's state has an incompatible format. This may happen when the file is corrupted (partial writes) or when the baker's state format changes between two versions.

Manually testing the MR

• Launch a baker in sandbox;
• Stop the baker;
• Modify the baking state file;
• Restart the baker;
• Notice the warning.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR