Improve cmake

Compile gtest as a subproject which is recommended and add a utility for downloading and building gtest source code

添加wrr的文档说明

include wrr in register_lb.png

add weighted round robin

增加wrr的说明

Rephrase automatic comments

to enable probot to close issues automatically.

readable

Fix typo in bounded_queue.h

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.