5.2.1.2. max-stale
这个指令用于放宽reuse条件
"max-stale"指令没有值时
If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its freshness lifetime
by no more than the specified number of seconds. If no value is
assigned to max-stale, then the client is willing to accept a stale
response of any age.
表示客户端接受任意current_age的response
"max-stale"指令有值时
If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its freshness lifetime
by no more than the specified number of seconds.
表示client接受满足如下条件的response
0 <= current_age <= lifetime + max-stale
