Move append_vec buffered reader overflow handling to a struct

[kskalski]

Problem

Ownership, handling and capacity management of overflow buffer for filling contiguous account data during full accounts-db scan is interleaved between buffered_reader and append_vec modules.
This:

• is a bit hard to read and to verify optimal handling of overflow capacity
• prevents reader from being shared across calls to scan_accounts_stored_meta
• pollutes the API that provides contiguous slices of account bytes

Summary of Changes

• new struct BufReaderWithOverflow is added that encapsulates the dynamically allocated overflow buffer with capacity limits
• overflow buffer capacity is additionally capped by file size
• fill_buf_required_or_overflow function is removed and ContiguousBufFileRead trait is renamed to RequiredLenBufRead
• FileBufRead trait is separated from ContiguousBufFileRead to independently provide file offset tracking (this is also going to support multi-file navigation needed to share buffer across files)
• overflow buffer filling functionality is moved to BufReaderWithOverflow and BufferedReader's Read impl, which fills provided buffer with existing valid bytes + read directly from file into provided buf

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.36066% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.2%. Comparing base (82d3802) to head (10a2f0a).
Report is 23 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7058   +/-   ##
=======================================
  Coverage    83.2%    83.2%           
=======================================
  Files         853      853           
  Lines      374799   374882   +83     
=======================================
+ Hits       311897   311982   +85     
+ Misses      62902    62900    -2     

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[brooksprumo]

Attention: Patch coverage is 81.11888% with 27 lines in your changes missing coverage. Please review.

Are these code paths missing test coverage?

• https://app.codecov.io/gh/anza-xyz/agave/pull/7058#7f852b49f6e37e268e884d837dac1d84-R184
• https://app.codecov.io/gh/anza-xyz/agave/pull/7058#7f852b49f6e37e268e884d837dac1d84-R278
• https://app.codecov.io/gh/anza-xyz/agave/pull/7058#7f852b49f6e37e268e884d837dac1d84-R300
• https://app.codecov.io/gh/anza-xyz/agave/pull/7058#7f852b49f6e37e268e884d837dac1d84-R348

[brooksprumo]

Looking good. I still plan to do another pass.

Also, what's the plan/next steps for this code? Are there further refactorings coming?

[kskalski]

We are getting close to the target shape.
The next PR on top of this one is finally able to reuse the buffer across multiple storages scan: #7080

I know I've been going back and forth with this code and traits, but it's probably an easier to understand way than dumping both refactor and functional changes into a single PR.

Attention: Patch coverage is 81.11888% with 27 lines in your changes missing coverage. Please review.

Are these code paths missing test coverage?

* https://app.codecov.io/gh/anza-xyz/agave/pull/7058#7f852b49f6e37e268e884d837dac1d84-R184

* https://app.codecov.io/gh/anza-xyz/agave/pull/7058#7f852b49f6e37e268e884d837dac1d84-R278

* https://app.codecov.io/gh/anza-xyz/agave/pull/7058#7f852b49f6e37e268e884d837dac1d84-R300

* https://app.codecov.io/gh/anza-xyz/agave/pull/7058#7f852b49f6e37e268e884d837dac1d84-R348

Right, those code-paths are not used in prod and are there mostly for satisfying trait inheritance, but they should be correct - I added a test that mixes all 3 kinds of access patterns (Read, BufRead and RequiredLenBufRead), though I'm not sure how to access a new coverage report with the new changes...

[kskalski]

And the further work will be to implement FileBufRead using io_uring multi-file read-ahead reader (needs rebase and updates, but the basic functionality is already in #6878), which paired with BufReaderWithOverflow can be used for scanning full accounts storage data with background IO.

[brooksprumo]

@kskalski went over the PR with me on zoom. Thanks!