kernel/sequencer: compute kernel state

Context

When the kernel contains too many old messages in the delayed inbox, the kernel should switch to the Fallback state and start processing messages itself.

This MR defines:

• what is the state of the kernel sequencer: Fallback or Sequenced.
• how to update the state when there is a too old message in the delayed inbox
• when to update the state: on each SoL

The next MR is: !9080 (merged)

Manually testing the MR

cd src/kernel_sequencer
make test

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