Draft: kernel/sequencer: process messages on fallback mode

Context

When the delayed inbox has too many messages, the state of the kernel switch to State::Fallback, in this mode, the delayed inbox should be automatically emptied by the kernel.

This MR introduces a mechanism to automatically empty the delayed inbox.

Previous MR: !9141 (closed)

Manually testing the MR

cd src/kernel_sequencer
make check && make build && 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