使用example中的例子验证熔断功能(可选熔断和连接超时熔断)

**Describe the bug (描述bug)**
基于example中的例子验证可选熔断(根据失败率）
原始代码：https://github.com/52coder/incubator-brpc/tree/master/example/asynchronous_echo_c%2B%2B
client.cpp修改点：
https://github.com/52coder/incubator-brpc/blob/master/example/asynchronous_echo_c%2B%2B/client.cpp#L65
添加：
options.enable_circuit_breaker = true;
L30行修改为rr :
DEFINE_string(load_balancer, "rr", "The algorithm for load balancing");

server.cpp在改动前，先编译出来echo_server,然后在https://github.com/52coder/incubator-brpc/blob/master/example/asynchronous_echo_c%2B%2B/server.cpp#L63前面增加如下代码：
cntl->SetFailed(brpc::EREQUEST, "Fail to parse request");然后编译出二进制echo_server_fail，这里的想法是生成一个100%失败的server触发熔断。

**To Reproduce (复现方法)**
验证方法，三个终端下分别执行
./echo_server --port=8002
./echo_server --port=8003
./echo_server_fail --port=8001

client端运行：
./echo_client --server="list://192.168.49.1:8001,192.168.49.1:8002,192.168.49.1:8003"

**Expected behavior (期望行为)**
期望行为是由于client端开启了熔断，其中一个server一直返回失败，所以应该会触发熔断才对，实际结果是跑了一晚上，一直出错的那个server并未摘除
`
I0803 00:26:21.137101 1872894 client.cpp:45] Received response from 192.168.49.1:8002: hello world (attached=bar) latency=603us

I0803 00:26:22.137842 1872886 client.cpp:45] Received response from 192.168.49.1:8003: hello world (attached=bar) latency=734us

W0803 00:26:23.138861 1872894 client.cpp:42] Fail to send EchoRequest, [E1003][127.0.1.1:8001][E1003]Fail to parse request

I0803 00:26:24.139042 1872886 client.cpp:45] Received response from 192.168.49.1:8002: hello world (attached=bar) latency=590us

I0803 00:26:25.139412 1872894 client.cpp:45] Received response from 192.168.49.1:8003: hello world (attached=bar) latency=639us

W0803 00:26:26.140028 1872886 client.cpp:42] Fail to send EchoRequest, [E1003][127.0.1.1:8001][E1003]Fail to parse request

I0803 00:26:27.140468 1872894 client.cpp:45] Received response from 192.168.49.1:8002: hello world (attached=bar) latency=733us

I0803 00:26:28.141491 1872886 client.cpp:45] Received response from 192.168.49.1:8003: hello world (attached=bar) latency=739us

W0803 00:26:29.142124 1872894 client.cpp:42] Fail to send EchoRequest, [E1003][127.0.1.1:8001][E1003]Fail to parse request

I0803 00:26:30.142181 1872886 client.cpp:45] Received response from 192.168.49.1:8002: hello world (attached=bar) latency=484us

I0803 00:26:31.142817 1872894 client.cpp:45] Received response from 192.168.49.1:8003: hello world (attached=bar) latency=737us

W0803 00:26:32.143875 1872886 client.cpp:42] Fail to send EchoRequest, [E1003][127.0.1.1:8001][E1003]Fail to parse request

I0803 00:26:33.144834 1872894 client.cpp:45] Received response from 192.168.49.1:8002: hello world (attached=bar) latency=748us

I0803 00:26:34.145134 1872886 client.cpp:45] Received response from 192.168.49.1:8003: hello world (attached=bar) latency=676us

W0803 00:26:35.145437 1872894 client.cpp:42] Fail to send EchoRequest, [E1003][127.0.1.1:8001][E1003]Fail to parse request

I0803 00:26:36.145931 1872886 client.cpp:45] Received response from 192.168.49.1:8002: hello world (attached=bar) latency=739us

`

**Versions (各种版本)**
OS:centos7
Compiler:
brpc:最新版
protobuf:
熔断参数均使用默认值：https://github.com/apache/incubator-brpc/blob/master/src/brpc/circuit_breaker.cpp#L32
代码看了Controller::Call::OnComplete---->FeedbackCircuitBreaker。
**Additional context/screenshots (更多上下文/截图)**
另外连接超时的熔断也未构造出来，设置连接超时时间1ms,rpc超时时间5ms,看服务中打印超时1008错误，但实际未触发熔断。

Reproduce the behavior with example/asynchronous_echo_c++/client.cpp and server.cpp using the three-server commands, including the failing server and the timeout settings. Then read src/brpc/circuit_breaker.cpp and follow Controller::Call::OnComplete to FeedbackCircuitBreaker. Done means the failure-rate and connection-timeout cases produce the expected circuit-breaker behavior rather than repeatedly selecting the failing server.

Written by the indexing model from the issue text.