iks: get_latest command added

+ command return information about most recent probe of specified param
  of specified probe type. Output is timestamp of valid probe or error
  if no values for this param was found. Example:

    <- get_latest 0 "Status:Sensors:GPU Temperature" 10m
    -> k 0 1401706800
    <- get_latest 0 "Status:Sensors:GPU Temperature" 10s
    -> k 0 1401706950
    <- get_latest 0 "Status:Sensors:GPU Temperature" 1h
    -> k 0 1401706800
    <- get_latest 0 "Status:Sensors:GPU Temperature" 1d
    -> k 0 1401660000