一些注释说明改善.

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添加一个final_url()接口用于获得最终所请求的url信息.

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为multi_download添加request_option设置.

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改善multi_download的实现.

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改善一些multi_download实现.

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添加一些settings相关的定义.

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规范书写, 注意大小写.

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为http_options添加详细说明.

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multi_download修正在一些确定性错误后不再尝试重连.

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设置链接为done状态.

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修改命名, 避免名字冲突.

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rename in_range to check_range.

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add fetch_data function, no implement.

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修正注解.

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修正代码命名.

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完善说明

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添加get_range实现.

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添加inverse实现.

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1. 启用m_download_point, 可以根据fetch_data来下载指定偏移位置的数据.
2. 修正AVHTTP_DISABLE_THREAD丢失在allocate_range函数, 可能导致在启用
   AVHTTP_DISABLE_THREAD时, 单线程执行io_service的情况下还使用锁. 正
   常来讲, 在一个单线程执行io_service时, 可以启用AVHTTP_DISABLE_THREAD
   从而避免锁, 提高系统效率.
   还可以考虑使用strand来避免锁, 在多线程执行io_service的情况下.

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实现fetch_data操作.

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