Last time data

Fixes #4786 : display time last data is updated from each agent 

I create a cache class that storages the last time when data is updated and also the message of success or error (if there is a problem when tried to update data). AgentStatusPoller is now sending additional information ('update_info') to wmstats:

Before:
agentInfo -> {'status': 'ok', 'agent_url': 'cmssrv101.fnal.gov:9999', 'agent_team': 'team1,team2,cmsdataops', 'down_component_detail': [], 'down_components': [], 'drain_mode': False, 'agent': 'WMAgentCommissioning'}

Now 
agentInfo -> {'status': 'ok', 'agent_url': 'cmssrv101.fnal.gov:9999', 'agent_team': 'team1,team2,cmsdataops', 'down_component_detail': [], 'down_components': [], 'drain_mode': False, 'update_info': {'message': 'Data upload was successful', 'last_time': 1380744254}, 'agent': 'WMAgentCommissioning'}

@ticoann Please let me know if there is any comment.