mongodb client 支持

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**
brpc支持请求mongodb服务，支持副本集。

**Describe the solution you'd like (描述你期望的解决方法)**
实现[MongoDB Wire Protocol](https://www.mongodb.com/docs/manual/reference/mongodb-wire-protocol/)，使用Request Opcode `OP_MSG`。添加InsertRequest/Response，QueryRequest/Response，GetMoreRequest/Response（游标操作），CountRequest/Response，DeleteRequest/Response，UpdateRequest/Response，FindAndModifyRequest/Response protobuf定义，`PackRequest`转换为Mongo OP_MSG，`ProcessResponse`将解析Mongo OP_MSG，转换为pb。
副本集功能的支持，添加一个`NamingService`通过`replSetGetStatus`获取副本集中的各个节点的状态，返回`PRIMARY`节点。

**Describe alternatives you've considered (描述你想到的折衷方案)**
MongoDB文档没有详细描述CRUD操作的协议，因此可以通过编写MongoDB CRUD测试程序抓包模仿实现。

**Additional context/screenshots (更多上下文/截图)**
现在已经实现了mongo insert, find, getMore, count, delete, update, findAndModify操作，能够支持副本集（只会使用PRIMARY节点）

Search the brpc repository for the MongoDB client implementation described in the issue, starting with its MongoDB Wire Protocol OP_MSG handling. Check whether insert, find, getMore, count, delete, update, findAndModify, and replica-set PRIMARY selection are already present. Done means these operations and replica-set support are implemented and validated by CRUD and replica-set tests.

Written by the indexing model from the issue text.