1. Change `brpc_socket_max_unwritten_bytes` to 1GB
This can make the system more fault-tolerant.
Especially in the case of high system load, try to reduce EOVERCROWDED errors.
2. Change `brpc_max_body_size` to 3GB
To handle some large object such as bitmap or string.
Close related #7334
1. Fix bug describe in [Bug] show frontends cause FE oom #7334
2. Fix error of CurrentConnected fields in show frontends result.
3. Add more FAQ
