Move query-scoped data to a separate data type accessed by Rows.Finish()

[flimzy]

Closes #552

[codecov]

Codecov Report

Merging #554 (a057c6d) into master (b5116f3) will increase coverage by 0.06%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master     #554      +/-   ##
==========================================
+ Coverage   87.13%   87.19%   +0.06%     
==========================================
  Files          21       21              
  Lines        1158     1156       -2     
==========================================
- Hits         1009     1008       -1     
+ Misses        105      103       -2     
- Partials       44       45       +1     

Impacted Files	Coverage Δ
row.go	13.88% <0.00%> (-0.82%)	⬇️
rows.go	84.67% <92.85%> (+1.86%)	⬆️

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