Remove startPosInChunk from append() and fix incorrect numValues

[ray6080]

Both ColumnChunk::append interfaces take a startPosInChunk as input, which is very confusing, as append should auto add value to its end, whose position is indicated by numValues. And this interface actually hides potentially annoying bugs related to incorrect setting of numValues.
This PR removes the param startPosInChunk and fix incorrect numValues inside NullColumnChunk.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (e393e33) 91.16% compared to head (f8458fe) 91.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2387      +/-   ##
==========================================
+ Coverage   91.16%   91.17%   +0.01%     
==========================================
  Files        1012     1012              
  Lines       35923    35933      +10     
==========================================
+ Hits        32748    32761      +13     
+ Misses       3175     3172       -3     

Files	Coverage Δ
src/include/storage/store/string_column_chunk.h	100.00% <ø> (ø)
src/include/storage/store/struct_column_chunk.h	100.00% <ø> (ø)
src/include/storage/store/var_list_column_chunk.h	90.90% <100.00%> (ø)
src/storage/local_table.cpp	94.09% <100.00%> (ø)
src/storage/store/node_group.cpp	100.00% <100.00%> (ø)
src/storage/store/string_column_chunk.cpp	84.41% <100.00%> (ø)
src/storage/store/struct_column_chunk.cpp	70.73% <100.00%> (ø)
src/storage/store/var_list_column_chunk.cpp	96.62% <100.00%> (+0.03%)	⬆️
src/include/storage/store/column_chunk.h	97.61% <80.00%> (-2.39%)	⬇️
src/storage/store/column_chunk.cpp	92.64% <82.35%> (-0.27%)	⬇️

... and 4 files with indirect coverage changes

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