Replace link_up timers with CooldownManager

[Ktmi]

When a link is activated, it is preferred that the link not immediately enter the link up state due to link flapping. Currently, the logic for handling this is spread throughout the topology napp, making it difficult to modify. Additionally, it does not support the new interruptions framework. To make this system easier to manage, I suggest moving it to its own dedicated manager. Additionally, to properly support the framework, there should be a separate thread which periodically checks which link cool downs have ended in order to generate interruption events as appropriate.

This issues is based on the following discussion #136 (comment)