ERROR: GetTransport of:xxx get timed_mutex timeout 解决办法

问题：
多线程并发调用 producer->send() 很容易得到错误消息：GetTransport of:xxx get timed_mutex timeout 除了libevent的版本问题，还有一个原因是gcc版本的bug

原因：
if (!lock.try_lock_for(std::chrono::seconds(m_tcpTransportTryLockTimeout))) {
LOG_ERROR("GetTransport of:%s get timed_mutex timeout", addr.c_str());
std::shared_ptr pTcp;
return pTcp;
}

gcc4.8.5的情况下，try_lock_for会立即返回，并不会等待期待的m_tcpTransportTryLockTimeout时长。 升级到4.9.0之后即可解决。

**std::timed_mutex::try_lock_for bug： https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54562**

@ShannonDing 建议在首页说明使用gcc版本应该高于等于4.9.0

Start by locating the project's homepage or installation requirements documentation mentioned in the issue. Document that GCC 4.9.0 or newer is required, including the reported std::timed_mutex::try_lock_for limitation; the work is done when the requirement is visible to users.

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