Data inconsistency problems

Hi,

Using your library I ran into some data inconsistency problems, specifically when two requests try to delete/insert the same data (for instance when using AJAX requests).

At the moment clearTriplet() and storeTriplet() are called sequently and that may cause problem in case of many requests.

To deal with the issue, I think it would be good to create replaceTriplet() method in StorageInterface to have ability to use locks/transaction depending on the storage in use.

Please merge if you're happy with it.

Best,
Bartosz