fix(storage): Increase AsyncWriter default MinLwmValue to avoid frequent flushes

[shubham-up-47]

The AsyncWriter was using small values of LWM (256 KiB) and HWM (512 KiB). This caused the writer to flush data far too frequently, creating a bottleneck,

• LWM i.e. Low Water Mark: When the write buffer size is equal to or greater than this value, the SDK code sets flush_=true and prepares for flush.
• HWM i.e. High Water Mark: When the write buffer size is equal to or greater than this value, writes are paused until the buffer size reaches below the LWM.

This PR increases the default MinLwmValue from 256 KiB to 16 MiB which significantly reduce the number of flush operations, allowing the AsyncWriter to batch more data and achieve much higher throughput. The new default is chosen based on Nokoro test results which demonstrated optimal performance at 16 MiB,

• 4 MiB LWM: ~275 MiB/s throughput
• 8 MiB LWM: ~470 MiB/s throughput
• 16 MiB LWM: ~603 MiB/s throughput
• 32 MiB LWM: ~305 MiB/s throughput

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.04%. Comparing base (3962f01) to head (5189b9d).
⚠️ Report is 108 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15552   +/-   ##
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  Coverage   93.04%   93.04%           
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  Files        2408     2408           
  Lines      220145   220145           
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  Hits       204829   204829           
  Misses      15316    15316           

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[scotthart]

@scotthart reviewed 1 of 1 files at r1, 3 of 3 files at r6, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @shubham-up-47)