Introduce QoS

[hackerwins]

Description:

Yorkie is exposed directly to external, which puts it at risk of being overwhelmed by resources by unspecified clients.

The Yorkie PushPull API can contain a lot of data in one request. In addition to the simple number of requests, it is necessary to design for the amount of data contained in the request. It may be helpful to refer to the ETCD kv RPC QoS proposal when designing.

• ETCD kv RPC QoS: kv RPC QoS etcd-io/etcd#8483.

Why:

• In fact, expensive requests may be common. Some newbies will not follow the best practices of Yorkie.