asynchronous health checking

SSL supports NS & connection_group support

issue#346 move #include out of namespace butil

merge some bugfix from baidu

-Add cntl->set_pb_jsonify_empty_array(bool). Convert the repeated field that has no entry to a empty array of json in HTTP response.
-Change log level in StreamWrite from FATAL to Warning when call
-StreamWrite after socket closed
-Fix attachment forward for baidu-rpc combination calls
-Fix hulurpc user data/attachment forward; Fix parallel request attachment forward
-Return Error if parse JsonString failed

- Add cntl->set_pb_jsonify_empty_array(bool). Convert the repeated field that has no entry to a empty array of json in HTTP response.

add GetSockOption to controller; fix some typo

Auto concurrency limiter

 change the backlog of listen() from INT_MAX to 65535

change the backlog of listen() from INT_MAX to 65535 due to the reason described in https://codereview.appspot.com/7480046/

update brpc to gydong/brpc