Need cleanup of agents data for agents that were removed already

[AlexeyKasatkin]

For now, data about all the agents since netchecker application deployment remains in TPR even if some agents were removed already. Need to make periodic cleanup of agents data for such agents in TPR (and in server memory if required).

[AlexeyKasatkin]

This issue is valid for TPR case only (no issue when working with etcd).