[dea-ads] remove/forget DEAs that stop advertising themselves

[drnic]

If a DEA is dead and/or is being resurrected, dea-ads continues to display the last stats it remembers; rather than negating the entire dea from the list to reflect that it is missing.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. You can view the current status of your issue at: https://www.pivotaltracker.com/story/show/76338250.