Add an extra field to the proxy namespace statistics to contain the last updated timestamp

[jtorreke]

The back story is that each volumedriver now has 2 proxies deployed. Each proxy contains statistics for a given set namespaces and without resetting the statistics, it will forever show the same statistics if a volume would move to another node/volumedriver and nothing would ever happen on a namespace on that proxy.
A possible solution would be to add a timestamp to tell us when the statistics were last updated or when the last get/put/delete/whatever was executed for that namespace on that proxy and give us an idea on how relevant the returned statistics are.