List of links (topology) should be in core and not in topology

[ajoaoff]

Original issue opened by @ajoaoff at https://github.com/kytos/kytos/issues/1064.

The list of links must be accessible by all applications, like the list of switches and interfaces. Although the topology is discovered by the kytos/topology NApp, the results should be stored in the core. The NApp kytos/of_core deals with switch connections, but the data is stored in the core.

[viniarck]

Related issues:

https://github.com/kytos-ng/maintenance/issues/90

https://github.com/kytos-ng/mef_eline/issues/464

[viniarck]

Quick summary of major points to be moved and refactored on both kytos and topology:

• Move _get_link_or_create(endpoint_a, endpoint_b) to kytos.core.controller.get_link_or_create(endpoint_a: Interface, endpoint_b: Interface)
• Move topology self.links dict to Controller.links
• Move topology self._links_lock to Controller._links_lock.
• Move topology get_link_from_interface(interface) to kytos.core.controller.get_link_from_interface(interface: Interface) -> Optional[Link]. This method should be thread safe using Controller._links_lock.
• Add a new method get_link(link_id: str) -> Optional[Link]
• Refactor all topology self.links usage to use kytos.core.controller.links instead, including preserving the shallow copies
• Refactor topology self._links_lock to use a shallow copy where it's iterating over links.
• Refactor topology on_interruption_start to use another local lock something like with self._interruption_lock each start event needs to be processed sequentially, and on_interruption_end to also use the same lock.
• Add a new method on kytos.core.controller.remove_link(link_id: str) -> Optional[Link]

All topology DB writes will still be managed by topology, only the Link instances refs should be moved to core just so certain memory reads becomes easier and avoid IO in many cases that it used to have in certain request calls.

topology endpoints will still stay the same, no compatibility broken except self.links which shouldn't accessed by other NApps, although maintenance is doing it https://github.com/kytos-ng/maintenance/issues/90, so that needs to be refactored too.