Recycles slices allocated by OwnedImpl back to the thread-local freelist

[birenroy]

This change allows mutable slices of default size (16KB) to be recycled
back to the thread-local freelist when they are drained and destroyed,
rather than being deleted. This reduces memory allocation frequency
for steady-state read operations.

We introduce a static recycle method to OwnedImplReservationSlicesOwnerMultiple
and call it from Slice destructor. To resolve circular dependency, the
Slice destructor is declared in the class and defined inline at the
end of the header. We also make Slice a friend of OwnedImpl to allow
access to the private nested class.

Added unit tests in buffer_test.cc to verify:

• Successful recycling and reuse of default-sized slices.
• Non-default-sized slices are not recycled.

This change was entirely generated by the Gemini AI model. I have read the code and
understand how it operates.

Benchmarks before:

--------------------------------------------------------------------------------
Benchmark                                      Time             CPU   Iterations
--------------------------------------------------------------------------------
bufferReserveCommitPartial/1                73.7 ns         73.6 ns      9787234
bufferReserveCommitPartial/4096             76.0 ns         76.0 ns      9377358
bufferReserveCommitPartial/16384            85.7 ns         85.7 ns      8166592
bufferReserveCommitPartial/65536             118 ns          118 ns      6029029
bufferReserveCommitPartial/131072            176 ns          176 ns      3468302
bufferReserveCommitMultiBuffer/16384         775 ns          775 ns       925720
bufferReserveCommitMultiBuffer/65536        2182 ns         2181 ns       327569
bufferReserveCommitMultiBuffer/131072       4016 ns         4014 ns       181183

Benchmarks after:

--------------------------------------------------------------------------------                  
Benchmark                                      Time             CPU   Iterations                  
--------------------------------------------------------------------------------                  
bufferReserveCommitPartial/1                74.0 ns         74.0 ns      9710692
bufferReserveCommitPartial/4096             74.0 ns         73.9 ns      9479766                  
bufferReserveCommitPartial/16384            88.5 ns         88.5 ns      7909427                  
bufferReserveCommitPartial/65536             115 ns          115 ns      6097813                  
bufferReserveCommitPartial/131072            168 ns          168 ns      4068071                  
bufferReserveCommitMultiBuffer/16384         687 ns          687 ns       927754                                                                                                                     
bufferReserveCommitMultiBuffer/65536        1839 ns         1839 ns       381225                                                                                                                     
bufferReserveCommitMultiBuffer/131072       3220 ns         3219 ns       222127

Commit Message:
Additional Description:
Risk Level:
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:

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Caused by: #46136 was opened by birenroy.

see: more, trace.