chunk/cache: disable refreshCacheKeys if interval < 0

[SandyXSD]

This is a very special case, which will totally disable building index for the cached keys. Users have to be make sure by themselves that there is at lease one client building this index, so that it can upload staging blocks (if writeback is enabled) and do the cleanup of cache directory.

[codecov-commenter]

Codecov Report

Base: 59.33% // Head: 59.34% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (a0f4931) compared to base (2caf355).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2865      +/-   ##
==========================================
+ Coverage   59.33%   59.34%   +0.01%     
==========================================
  Files         140      140              
  Lines       24331    24333       +2     
==========================================
+ Hits        14436    14441       +5     
+ Misses       8044     8038       -6     
- Partials     1851     1854       +3     

Impacted Files	Coverage Δ
pkg/chunk/disk_cache.go	74.95% <0.00%> (+0.69%)	⬆️
pkg/chunk/utils_unix.go	66.66% <0.00%> (-25.00%)	⬇️
pkg/vfs/writer.go	76.58% <0.00%> (-1.20%)	⬇️
pkg/meta/sql.go	64.40% <0.00%> (-0.34%)	⬇️
pkg/vfs/reader.go	79.20% <0.00%> (+0.21%)	⬆️
pkg/meta/tkv.go	75.48% <0.00%> (+0.45%)	⬆️
pkg/meta/tkv_mem.go	87.32% <0.00%> (+0.70%)	⬆️
pkg/fuse/context.go	75.00% <0.00%> (+10.71%)	⬆️

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