[REST] Result generators do not need to query table schema

Now, the RowResultGenerator and the ScanerResultGenerator will fit the column family if the request doesn't contain any column info.

      if (rowspec.hasColumns()) {
        //addColumn for get or scan
      } else {
        for (HColumnDescriptor family: 
            table.getTableDescriptor().getFamilies()) {
          scan/get.addFamily(family.getName());
        }
      }

The table.getTableDescriptor() will cost 10+ milliseconds in our hbase cluster each request. We can remove these code because the server will auto add the columns.