core/rawdb: fsync the index file after each freezer write

[ucwong]

No description provided.

[codecov]

Codecov Report

Merging #1804 (21a5acd) into master (ce2c5b3) will decrease coverage by 0.02%.
Report is 373 commits behind head on master.
The diff coverage is 6.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1804      +/-   ##
==========================================
- Coverage   52.72%   52.70%   -0.02%     
==========================================
  Files         485      485              
  Lines       60529    60538       +9     
==========================================
- Hits        31915    31909       -6     
- Misses      26156    26181      +25     
+ Partials     2458     2448      -10     

Files	Coverage Δ
core/rawdb/freezer_utils.go	51.85% <100.00%> (+1.85%)	⬆️
core/rawdb/ancient_utils.go	0.00% <0.00%> (ø)
core/rawdb/freezer_batch.go	80.39% <0.00%> (-3.29%)	⬇️
core/rawdb/freezer_table.go	74.02% <0.00%> (-0.93%)	⬇️

... and 12 files with indirect coverage changes