Flush staging files on close as much as possible

[polyrabbit]

Flush staging files as much as possible, making writeback safer to turn on.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 40.00000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 45.18%. Comparing base (a8cb0fc) to head (1783770). Report is 7 commits behind head on main.

:exclamation: Current head 1783770 differs from pull request most recent head 3ae32ef

Please upload reports for the commit 3ae32ef to get more accurate results.

Files	Patch %	Lines
pkg/chunk/cached_store.go	37.50%	15 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4965       +/-   ##
===========================================
+ Coverage   24.97%   45.18%   +20.21%     
===========================================
  Files          25      159      +134     
  Lines       16045    43185    +27140     
===========================================
+ Hits         4007    19515    +15508     
- Misses      11549    21307     +9758     
- Partials      489     2363     +1874     

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[jiefenghuang]

I think this is not an essential requirement for all writeback scenarios It would be more appropriate to place it in a separate command as a tool.