Allow calling internal methods also as notifications

[cmaglie]

Motivation

This is mainly required to allow sending messages to the monitor (mon/write) using a notification instead of a request. This change should increase the throughput of this debug channel.

Change description

The router's internal methods, expect $/register and $/reset, can now be invoked either as Request or as Notification.
If a client opts to use the Notification:

• It will lose the return/error values.
• It will not know when the action has been completed (the notification returns immediately, while a request will wait until the action completes).

Additional Notes

Reviewer checklist

• PR addresses a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.

[eigen-value]

LGTM