EVCs switchover stability

[ajoaoff]

Original issue opened by @viniarck at kytos#40.

This issue addresses this point: Sometimes the network could be unstable for several reasons, such as a bad fiber or other bad hardware, so, before switching over network paths we should be able to tell if the path is stable. As discussed on #26, networking vendors typically implement this functionality with a timer, so if in a specified time period no link flap events have occurred then it's considered stable to switch to.

Points to be discussed:

1 - The stability functionality should be applied to both primary and backup paths, so all paths will have a method to check if it's stable before the EVC actually switches to it. This is the most complete implementation, that will even not allow the primary path to switch over the backup path if just a quick link flap happened.
2 - The stability functionality should be applied only when the backup path is switching over to the primary path. This is a partial implementation. So any flap in the primary path will result in switching over immediately to the backup path.