prevent indefinite defer-close by checking last_active_time

[jenrryyou]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

在 SocketMap 里决定“是否进入 defer_close 倒计时”时，不再无条件
defer，而是先看该连接最近一次读/写的活跃时间。

• 具体改动：当 ref_count 归零准备移除连接时，只有在“连接最近仍有活跃（last_active_time_us 距今不超过 defer_close_second）”的情况下，才把 no_ref_us
置为当前时间、开始倒计时；如果连接其实已经闲置超过 defer_close_second，则直接从 socket_map 删除并释放 socket 引用（src/brpc/socket_map.cpp:293）。
• 意图：避免老 socket 在“channel 频繁创建/析构导致 ref_count 归零又很快被重新引用”的场景下反复刷新倒计时，从而把一个已经长期不活跃/可能已失败的连接在
socket_map 里拖很久，导致新连接迟迟不创建、请求持续复用老连接并依赖其健康检查重连（进而在重连慢时持续失败）。

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[wwbmmm]

You have changed the default behavior of SocketMap. Although it may be a better practice, it may cause some side effects to existing users. I suggest to add a new gflag to control this behavior, and update the document about defer_close_second.