brpc 访问etcd的restful接口的问题

我想尝试用brpc来访问etcd的kv服务，实现了一个put key value的功能，demo代码如下，功能是正常的，但是我想用brpc实现watch功能，就出现问题了。
```
void PutKeyValue(brpc::Channel& channel) {
brpc::Controller cntl;
brpc::URI& uri = cntl.http_request().uri();
uri.set_path("/v3/kv/put");
cntl.http_request().set_method(brpc::HTTP_METHOD_POST);
butil::IOBufBuilder os;
os << "{\"key\":\"Zm9v\",\"value\":\"YmFy\"}";
os.move_to(cntl.request_attachment());

channel.CallMethod(NULL, &cntl, NULL, NULL, NULL);
if (cntl.Failed()) {
std::cerr << cntl.ErrorText() << std::endl;
return;
}
std::cout << cntl.response_attachment() << std::endl;
}
```
watch的demo代码如下, 调用CallMethod后，就一直阻塞在这个调用这里了。 这个watch机制应该是http的 chunked mode， 不知道brpc client应该如何处理这种chunked mode？

```
void Watch(brpc::Channel& channel) {
brpc::Controller cntl;
cntl.http_request().uri().set_path("/v3/watch");
cntl.http_request().set_method(brpc::HTTP_METHOD_POST);
butil::IOBufBuilder os;
os << "{\"create_request\": {\"key\":\"Zm9v\"}}";
os.move_to(cntl.request_attachment());

channel.CallMethod(NULL, &cntl, NULL, NULL, NULL);
if (cntl.Failed()) {
std::cerr << cntl.ErrorText() << std::endl;
return;
}
std::cout << cntl.response_attachment() << std::endl;
while (1) {sleep(1111111);}
}
```

Start with the brpc::Channel and brpc::Controller usage shown for /v3/kv/put and /v3/watch, then inspect how CallMethod handles the HTTP response attachment. Reproduce the /v3/watch request against etcd and determine how its chunked response should be consumed; done means the watch request can receive streamed events without blocking indefinitely.

Written by the indexing model from the issue text.