Add a mempool component to the protocol

Context

This MR adds a mempool component to the protocol with the capability of adding/removing operations and merging two mempool while guaranteeing the property that all operations contained in it are valid and commutable modulo the correct ordering of the operations in the block and the inclusion of sufficient endorsements/preendorsements.

In depth explanations are provided in the module's interface.

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