CSR header: seprating offset and length

[ray6080]

This is the precursor of the CSR update optimization. Only refactors existing code.

1. Add length for each csr list. The offsets, together with lengths constitute CSRHeader. The change is mainly due to that we need to leave gaps between csr lists, and only keeping offsets is not enough to tell the length of each csr list.
2. Separate CSR specific rel table data to a subclass CSRRelTableData of RelTableData.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (dd0efd3) 93.24% compared to head (c94a03a) 93.25%.

❗ Current head c94a03a differs from pull request most recent head 0ce0c87. Consider uploading reports for the commit 0ce0c87 to get more accurate results

Files	Patch %	Lines
src/storage/store/rel_table_data.cpp	98.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2601      +/-   ##
==========================================
+ Coverage   93.24%   93.25%   +0.01%     
==========================================
  Files        1037     1034       -3     
  Lines       38815    38723      -92     
==========================================
- Hits        36193    36113      -80     
+ Misses       2622     2610      -12     

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