Add a cache mechanism in the protocol

Context

Please read the design document found here: https://hackmd.io/@yrg/SyBubvs5O

This MR corresponds to the first step of the development plan for cache. This MR:

• extends the environment to offer a caching mechanism to the protocol ;
• uses this mechanism to cache the script code.

Unit tests are being polished and will come in a follow-up 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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR