Aggregate baker's highwatermarks I/Os

Context

This MR improves the baker performances by reducing the number of on-disk writes when registering the consensus votes high watermarks. There is now only one update (i.e. read the file, decode the file, update the structure, write the structure in the file) instead of n where n is the number of consensus-vote producing bakers.

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