feat(stream): fast buffer freeing for large cache

[j2rong4cn]

添加配置选项 mmap_thresholdMB

默认为4 单位：MB
大于或等于该大小的[]byte将使用匿名mmap来达到快速释放内存占用的目的

[Copilot]

Pull Request Overview

This PR introduces a memory management optimization for large buffer allocations by adding support for anonymous mmap allocation. The feature enables faster memory freeing for large caches through a configurable threshold.

Key changes:

• Adds MinMmapAllocSize configuration option (default 4MB) to control when mmap allocation is used
• Replaces sync.Pool with a custom generic Pool[T] implementation for better type safety
• Implements mmap-based buffer allocation for buffers exceeding the threshold size

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
pkg/pool/pool.go	Introduces new generic pool implementation with capacity limits
internal/stream/util.go	Updates buffer pool to use custom pool and mmap allocation for large buffers
internal/stream/stream.go	Adds mmap allocation support for file stream caching
internal/net/request.go	Refactors Buf struct to use direct byte slice with mmap support instead of bytes.Buffer
internal/conf/var.go	Adds MinMmapAllocSize configuration variable
internal/conf/config.go	Adds MinMmapAllocSize field to Config struct with default value
internal/bootstrap/config.go	Initializes MinMmapAllocSize from configuration
internal/stream/stream_test.go	Updates test configuration and test name

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