Implement BufRead for accounts_db BufferedReader

[kskalski]

Problem

BufferedReader uses a custom API to implement the same access pattern as BufRead trait (though it also relaxes some of its constraints and adds a couple of extensions).
The custom API makes it harder to understand and reason about, but also it's harder to implement/substitute with an alternative implementation (e.g. io_uring read-ahead buffered reader).

Summary of Changes

BufferedReader provides all the functionality of BufRead, so the custom API is converted to use of fill_buf and consume. The extra functionality like access to current offset and setting min required data is kept as pub API of BufferedReader (when alternative impl is available, those should be elevated to separete trait that will extend BufRead).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.83051% with 12 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@0de8717). Learn more about missing BASE report.
Report is 2 commits behind head on master.

Additional details and impacted files

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  Hits              ?   315498           
  Misses            ?    63052           
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[cpubot]

Great improvement, thank you!

[brooksprumo]