perf(quinn-proto): Avoid creating small Bytes when doing small writes via AsyncWrite::write

[rklaehn]

Description

Defers the creation of the Bytes a tiny bit more, so we can decide in the SendBufferData to either copy or create a Bytes.

For anything less than MAX_COMBINE, we will not create a Bytes at all.

Breaking Changes

None

Notes & open questions

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.88957% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.42%. Comparing base (38a7d15) to head (1b484db).

Files with missing lines	Patch %	Lines
quinn-proto/src/connection/send_buffer.rs	82.83%	23 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           main-iroh     #252      +/-   ##
=============================================
- Coverage      76.52%   76.42%   -0.11%     
=============================================
  Files             83       83              
  Lines          23018    23110      +92     
=============================================
+ Hits           17615    17662      +47     
- Misses          5403     5448      +45     

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