Split packet clearing schedule from packet worker

[ljoss17]

Closes: #4071

Description

This PR removes the packet clearing logic from the packet_cmd_worker.
The reason is to avoid blocking the worker if there is a huge amount of pending packets.

The packet_cmd_worker will not handle packet clearing anymore, instead an additional background task, clear_cmd_worker will schedule pending packets to be cleared. And the packets will be cleared by the when handle_execute_schedule is called.
A new configuration clear_limit is added in order to only clear up to a maximum of n packets every time clearing is triggered. The default value is 50.

There is an exception for Ordered channels, in which case the clearing is blocking and is done before relaying. This is because relaying without completing the packet clearing before would fail due to an incorrect packet sequence error.

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests: integration (for Hermes) or unit/mock tests (for modules).
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[romac]

Great work! Left a couple nits