meta: slowdown dump process to reduce memory usage

[davies]

close #4252

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (71fdb17) 55.73% compared to head (a9336db) 55.54%.

Files	Patch %	Lines
pkg/meta/tkv.go	16.66%	9 Missing and 1 partial ⚠️
pkg/meta/sql.go	33.33%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4253      +/-   ##
==========================================
- Coverage   55.73%   55.54%   -0.19%     
==========================================
  Files         153      120      -33     
  Lines       39516    31113    -8403     
==========================================
- Hits        22023    17281    -4742     
+ Misses      15022    11791    -3231     
+ Partials     2471     2041     -430     

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[github-actions]

Mutate Test Report
pkg/meta/load_dump_test.go-1-1: score:0.00 failed:0, passed:0, compile error:0, out of coverage:0, skip by comment:0, others:0
Job detail: https://github.com/juicedata/juicefs/actions/runs/7179829304/job/19550882724

Usage: https://github.com/juicedata/juicefs/blob/main/.github/scripts/mutate/how_to_use_mutate_test.md