Crash when network is down on Mac.

[ShannonDing]

1. send messages
2. shutdown wifi
3. open wifi
4. producer crash

libc++abi.dylib: terminating with uncaught exception of type rocketmq::MQClientException: msg: Failed to resolve  host name(nameserver.rocketmq.com): nodename nor servname provided, or not known,error:-1,in file </Users/shannon/OpenSource/Apache/rocketmq-client-cpp/src/transport/TcpTransport.cpp> line:108
Abort trap: 6

[ifplusor]

Same as #310