ContainerInfoHandler cannot deal with non-unique timestamps

[sarni]

The class ContainerInfoHandler is responsible for returning the last values for each virtual sensors as an xml file. But if several tuples have their timestamp equal to the max(timestamp), they will be merged and there will be duplicated fields with no way to group them.
(Moreover the webinput fields will be duplicated too)

Imported from: https://sourceforge.net/apps/trac/gsn/ticket/92

[ebiiii]

I implemented a simple solution that uses the primary key to ensure we always return the only last stream-element (which is the purpose of the ContainerInfoHandler). If we keep this solution, we can make some cleaning like removing the for loop...