Make BufferedReader usable on all platforms to support read of append_vec always through File

[kskalski]

Problem

BufferedReader is excluded from use on non-unix because read_at is a os::unix API, however there is similar API also available on windows, so we could extend its usage.

Summary of Changes

Narrow down platform-specific function call to read at offset and provide unix and windows implementation of it.

Notes:

• not-unix and not-windows implementation is not provided - a stub that panics could be provided, but the goal is to make BufferedReader universally available and catch any other platform at compile time
• windows implementation updates the internal file cursor, so file_io shouldn't be used when regular reads from file assume starting at 0 or last used offset

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.3%. Comparing base (f39ea99) to head (2f0ef5c).

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #6876     +/-   ##
=========================================
- Coverage    83.3%    83.3%   -0.1%     
=========================================
  Files         853      853             
  Lines      378508   378511      +3     
=========================================
- Hits       315482   315471     -11     
- Misses      63026    63040     +14     

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[brooksprumo]

I dunno... I don't think we should add any support for Windows for anything IMO. We don't actually support running a real validator on Windows, so adding code that requires support and testing doesn't seem like a great idea.

[kskalski]

This unifies the codebase and I was hoping to make this code an actual fallback for an even more linux-specific io_uring implementation of buffered reader.

Also I'm wondering it it would be possible to remove more code if this supports windows - e.g. mmap append vec is serving as fallback for not becing able to use BufferedReader

[brooksprumo]

[brooksprumo]

Adding @cpubot as a reviewer, since he's touched a lot of this code recently.

[cpubot]

Great