使用DynamicPartitionChannel时，request map的接口没有提供分片总数

[cdjingit]

Describe the bug (描述bug)

使用DynamicPartitionChannel时，各个片库的分片数是变化的, Request Map里只有channel_idx, 没有分片总数的信息。
https://github.com/apache/incubator-brpc/blob/master/src/brpc/parallel_channel.h#L94

virtual SubCall Map(int channel_index/*starting from 0*/,
                        const google::protobuf::MethodDescriptor* method,
                        const google::protobuf::Message* request,
                        google::protobuf::Message* response) = 0;

接口改为：
virtual SubCall Map(int channel_index/*starting from 0*/,
                        int channel_count,
                        const google::protobuf::MethodDescriptor* method,
                        const google::protobuf::Message* request,
                        google::protobuf::Message* response) = 0;

[wwbmmm]

是否可以改为

virtual SubCall Map(int channel_index/*starting from 0*/,
                        int /*channel_count*/,
                        const google::protobuf::MethodDescriptor* method,
                        const google::protobuf::Message* request,
                        google::protobuf::Message* response) {
    return Map(channel_index, method, request, response);
}

兼容一下现有代码

[cdjingit]

可以

[cdjingit]

#2057