Improve efficiency to solve the baker PoW challenge

Context

Not solving the proof of work challenge in a stupid way in octez baker allows to set a level of difficulty which would not be trivially achievable to a spammer because "it already takes time for the baker to reach".

This patch makes the baker 2x more efficient on my laptop and 3 time more efficient on my odroid C4.

Manually testing the MR

octez-client -p ProtoALpha bench 100 baking PoW challenges

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