CountdownEvent多bthread core dump问题

[qqwangxiaow]

Describe the bug (描述bug)
如果使用多bthread并发调用一个接口, 接口中会生成bthread::CountdownEvent wait()方法等待server返回, 会core
使用多pthread就不会
#0 0x000055fb32abc0c3 in std::__atomic_base::load (__m=std::memory_order_acquire, this=0x0) at /usr/include/c++/6/bits/atomic_base.h:396
#1 bthread::CountdownEvent::wait (this=this@entry=0x7f462c01bfb0) at /tmp/build-C6o1eq/brpc-1.5.0/src/bthread/countdown_event.cpp:60
#2 0x000055fb32a7652b in rsdk::client::RefCountClosure::wait (this=)
To Reproduce (复现方法)

Expected behavior (期望行为)

Versions (各种版本)
OS:
Compiler:
brpc:
protobuf:

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

[qqwangxiaow]

缺少 sinal_and_wait 这种方法 提前xigou了