streaming writes flush and sync flow

[ashmeenkaur]

Description

This PR includes code changes to:

• separate out flush and sync flow.
• file inode test to test for both sync and flush.
• integrate flush file and sync file for streaming writes and associated refactoring.

Link to the issue in case of a bug fix.

b/385064030

Testing details

1. Manual - NA
2. Unit tests - added
3. Integration tests - NA

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.18%. Comparing base (94eddd0) to head (36372bc).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2856      +/-   ##
==========================================
+ Coverage   76.04%   76.18%   +0.13%     
==========================================
  Files         118      118              
  Lines       16557    16587      +30     
==========================================
+ Hits        12591    12636      +45     
+ Misses       3433     3421      -12     
+ Partials      533      530       -3     

Flag	Coverage Δ
unittests	76.18% <100.00%> (+0.13%)	⬆️

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