Caching json statistics

Periodic thread now also dumps JSON statistics. All clients get cached JSON statistics via eblob_stat_json_get. This statistics and some part of it (like dstat and vfs) include timestamps when it was generated. eblob_stat_json_get also checks cached statistics lifetime and if it is greater than doubled timeout of json update, adds 'error' section to json. This section provides error information and tells that returned json is too old. This may occur because of some issue with backend's hardware.