meta: delay deletion of compacted slices

[SandyXSD]

close #1521

TODO:

• Dump & Load: need a new struct DumpedDelSlice
  • better not load the delSlices, because the whole sliceRefs is rebuild when loading entries, and it's dangerous to further decrease any of the refs. Instead we can just run gc --delete command after loading to clean up the leaked slices.
• Unit tests
• SQL: need a new table: delslices{chunkid, ts, blob}
• TKV: Lcccccccctttttttt -> blob

[codecov-commenter]

Codecov Report

Merging #1790 (7c81854) into main (c76cf44) will decrease coverage by 0.21%.
The diff coverage is 61.51%.

❗ Current head 7c81854 differs from pull request most recent head 8174d12. Consider uploading reports for the commit 8174d12 to get more accurate results

@@            Coverage Diff             @@
##             main    #1790      +/-   ##
==========================================
- Coverage   61.90%   61.69%   -0.22%     
==========================================
  Files         135      135              
  Lines       23011    23246     +235     
==========================================
+ Hits        14246    14342      +96     
- Misses       7171     7272     +101     
- Partials     1594     1632      +38     

Impacted Files	Coverage Δ
pkg/meta/base.go	69.82% <38.46%> (-1.87%)	⬇️
pkg/meta/redis.go	72.81% <55.85%> (-0.43%)	⬇️
pkg/meta/sql.go	66.50% <68.49%> (-0.55%)	⬇️
pkg/meta/tkv.go	76.14% <71.21%> (-0.32%)	⬇️
pkg/sync/sync.go	45.17% <100.00%> (+0.19%)	⬆️
pkg/fuse/context.go	56.75% <0.00%> (-10.82%)	⬇️
pkg/vfs/reader.go	80.29% <0.00%> (-4.38%)	⬇️
pkg/chunk/cached_store.go	76.05% <0.00%> (-2.20%)	⬇️
... and 3 more

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