Implement S/G IO for non-batched sends and eliminate more data copies

[cgutman]

Description

This PR implements scatter/gather I/O support in the platform socket code for non-batched sends. Because we can specify separate header and payload buffers, this allows us to eliminate all redundant copies of audio shards. The performance impact of this change alone is quite small, but it will reduce the complexity of the following PR that implements S/G for batched sends.

I also included a drive-by fix for another missing std::vector::reserve() call that was causing horrible performance when using std::copy() with std::back_iterator().

Testing checklist:

• Windows with FEC validation
• Mac with FEC validation
• Linux with FEC validation

Screenshot

Issues Fixed or Closed

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

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must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[codecov]

Codecov Report

Attention: Patch coverage is 1.61290% with 61 lines in your changes missing coverage. Please review.

Project coverage is 9.22%. Comparing base (b93756a) to head (729d538).

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #2867      +/-   ##
=========================================
- Coverage    9.23%   9.22%   -0.02%     
=========================================
  Files          97      97              
  Lines       17440   17451      +11     
  Branches     8314    8300      -14     
=========================================
- Hits         1611    1610       -1     
- Misses      12946   13122     +176     
+ Partials     2883    2719     -164     

Flag	Coverage Δ
Linux	6.99% <0.00%> (+<0.01%)	⬆️
Windows	4.53% <0.00%> (+<0.01%)	⬆️
macOS-12	10.31% <2.56%> (-0.01%)	⬇️
macOS-13	10.23% <2.56%> (-0.01%)	⬇️
macOS-14	10.54% <2.63%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/platform/common.h	39.74% <ø> (ø)
src/platform/macos/misc.mm	6.71% <9.09%> (-0.14%)	⬇️
src/platform/linux/misc.cpp	8.91% <0.00%> (-0.14%)	⬇️
src/platform/windows/misc.cpp	2.21% <0.00%> (-0.02%)	⬇️
src/stream.cpp	3.33% <0.00%> (-0.09%)	⬇️

... and 22 files with indirect coverage changes