Avoid using same opendal cache during compaction

[v0y4g3r]

What type of enhancement is this?

Performance

What does the enhancement do?

If compactor use the same opendal backend (and cache directory) as datanode, it may pollute the cache in opendal backend especially compaction legacy sst files.

Implementation challenges

Separate opendal backend in compactor and datanode. Backend in compactor should only read file cache but not write to it.

[v0y4g3r]

Due to the pending optimization on access layer cache, we can now remove the cache layer during compaction. Theoretically, this change won't affect compaction performance, in that most reads in compaction do not retrieve repeated ranges.