rw lock implementation

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**

写优先bthread读写锁的实现
**Describe the solution you'd like (描述你期望的解决方法)**

我们提供了一种实现，参考内核写优先的设计
**Describe alternatives you've considered (描述你想到的折衷方案)**

https://github.com/hairet/brpc/commit/b2f23243ba3127db82d7ee17e6caba40bead0ef3

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

Start by reviewing the referenced hairet/brpc commit and locating brpc's existing bthread read-write lock implementation. Clarify the intended kernel-inspired write-preference behavior and identify the relevant lock tests or validation path. Done should mean the proposed write-preferring implementation is integrated and its behavior is verified.

Written by the indexing model from the issue text.