[core] introduce Placeholder for Blob File Format#7889
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Purpose
This is the first part of #7881
Including:
a. At first, all data files will be divided according to max_seq_num
b. within each group, create a sequential reader to logically concat files and fill missing gaps. For example: If the full row range of normal files is [0, 100], but some group only have one file with range [20, 80], the output is: [0, 19] -> filled with placeholders; [20, 80] -> records from files; [81, 100] -> filled with placeholders.
c. create readers for each group, and read the blob from the max group whose value is NOT a placeholder.
The mechanism can be illustrated as below:
Tests
ITCase and Unit tests