Proto/DAL: memoize Dal.make

Context

This MR is an optimization of the DAL slot header publication operation. It consists in memoizing in the context the result of the DAL.make environment function which is computation-incentive because it loads a large file (the DAL SRS).

I don't know if this optimization is worth it given that there is a plan to split the SRS so that the protocol only loads a tiny part of it.

Manually testing the MR

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