stop using a local cache on TaskDistributionTool

A dictionary was used as cached to store test result path and
duration of tests. This was quite convenient, however this
dictionary was growing, and since this data was commited thouzands
time per day, this was making the zodb growing faster and faster.

So we now rely on catalog + tags to retrieve test results. We
also use int_index on test result line to order them by duration
time. We will surely increase the load of sql database (we might
need to add specific index), but this will avoid zodb using too
much space.