issue: https://github.com/milvus-io/milvus/issues/36182
* improved `Column.h` to make the code much more readable and
maintainable, and added detailed comments.
* fixed an issue where `ArrayColumn::NumRows()` always returns 0 when
the mmap backing storage is a file.
* removed unused `ColumnBase` constructors and unnecessary members so we
don't get confused.
* Updated `test_chunk_cache.cpp` to make the tests parameterized: to
test both mmap enabled and disabled. Added sparse field in the test to
add coverage.
* re-enabled test `Sealed::GetSparseVectorFromChunkCache`.
* But 2 other disabled tests `Sealed::WarmupChunkCache` and
`Sealed::GetVectorFromChunkCache` remain disabled, there seems to be
errors. @bigsheeper PTAL.
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Signed-off-by: Buqian Zheng <zhengbuqian@gmail.com>
8495bc6bbc