stream 兼容连接池（pooled/multi）。

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**
使用stream来管理一个单连接上多个用户态的连接后，当前的实现是不兼容连接池模式的（pooled/multi连接模式）。
stream和连接池并不是互斥的特性，希望可以兼容。
比如现在h2就不支持连接池模式。

**Describe the solution you'd like (描述你期望的解决方法)**

**Describe alternatives you've considered (描述你想到的折衷方案)**

**Additional context/screenshots (更多上下文/截图)**

Start by tracing brpc's stream handling and pooled/multi connection-mode paths, then compare the current h2 behavior with the single-connection case described in the issue. Done means stream can be used alongside pooled/multi connection pooling, including h2, without losing existing stream behavior.

Written by the indexing model from the issue text.