fix(store): check if link exists on write

[walldiss]

In case of unexpected shutdown file might be partially written, while link is not. This PR will ensure we that put will result in both link and file on success.

File validation will be tackled in another PR

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.

Project coverage is 46.56%. Comparing base (2469e7a) to head (7bd0266).
Report is 338 commits behind head on main.

Files with missing lines	Patch %	Lines
store/store.go	36.36%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3844      +/-   ##
==========================================
+ Coverage   44.83%   46.56%   +1.73%     
==========================================
  Files         265      314      +49     
  Lines       14620    18142    +3522     
==========================================
+ Hits         6555     8448    +1893     
- Misses       7313     8679    +1366     
- Partials      752     1015     +263     

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[Wondertan]

It asks for a test

[Wondertan]

For context: shwap-robus network has a few blocks missed in the storage, and the hypothesis is that they were missed during OOM leaving the Store in an inconsistent state(file exists, but link for it does not). This covers this possibility and we are yet to prove this is an issue

[cristaloleg]

SGTM