Moving to a different storage structure

* Initially, Storage was modelled as a Long->String map. This has the
  problem that I will have to convert the long back into a ISO8601
  timestamp. This would involve a O(n) operation.

* Now moving to a PCacheTimestamp->String map. This means that I can
  additionally store the ISO8601 timestamp and also override the
  hashcode() and the equals() method of PCacheTimestamp to return the
  timestamp in miliseconds (the previous long value)  when it is being
  used in the map. This way I *think* I can escape the huge O(n)
  operation to convert the long to the timestamp