Use shim for legacy client read_rows

[daniel-sanche]

From go/bigtable_legacy_deprecation:

Old API

• The Table class exposed a read_row and read_rows method
  read_row simply calls read_rows under the hood, with a single row_key argument
• Read_rows creates and returns PartialRowsData instance, which can be iterated to return PartialRowData (no s) instances for each row in the query
  • The row merging logic is all contained within PartialRowsData, using internal _RowMerger and _ReadRowsRequestManager instances

Replacement

• We can replace the PartialRowsData class with a new subclass, that instead delegates row merging logic to the _ReadRowsOperation class provided by the new data client