weighted round robin load balancer

Add LOG_AT macro when linking with glog

1. Call done directly in StartConnect to avoid race with OnConnected in StopConnect
2. Dangling pointer in OnServerStreamCreated
3. Invalid _chunk_stream_id in closeStream and deleteStream

little change for algorithm: each tls has the same stride, but a different random beginning position of server list.

add SIGTERM handle for brpc controller