EVM/Sequencer: produce block at regular interval

Context

This merge request introduces the block producing of the sequencer. The routine used in the tx pool is now based on the current mode, therefore we are free to chose at which interval we want to publish a block. The first implementation is obviously naive because it doesn't account for the time spent in the block producing, but we'll get back to this later.

The merge request also contain a lot of noise to run an e2e test for the sequencer, but it is finally running!

Manually testing the MR

I really suggest you checkout the branch and run the tezt test:

tezt -f evm_sequencer.ml --verbose

You'll be able to see what the sequencer logs, how to run it, kill etc.

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