Faster archiving of aggregated reports, also performance imprvovements in general

* Store subtables in chunks of 100 subtables per blob. Those 100 subtables are stored
  serialized as an array: array($subtableID => subtableBlob). The first 100 subtables are
  stored in "chunk_0", the next 100 subtables are stored in "chunk_1", ...
* Subtable Ids are now consecutive from 1 to X
* We do no longer serialize the whole Row instance when archiving, instead we only
  serialize the Row's array which contains columns, metadata and datatable. This is not
  only more efficient but allows us to refactor the Row instance in the future (although
  we will always have to be BC)
* Faster row implementation: Columns, Metadata and Subtables access is much faster now