Richer progress report

[driv3r]

Added

• PaginationKey now includes column name

Changed

• Use PaginationKey instead of raw uint64 for progress report. This means that table progress report will
  include not raw value, but a whole PaginationKey object, i.e.

  {
    "type": "uint64",
    "column": "id",
    "value": 999
  }

  which will be in line with the format of what we have in the state dump
• Refactored MarshalPaginationKey function into MarshalJSON implementations, so there's less work during progress serialization