feat(interactive): Introduce proxy_server into interactive

[zhanglei1949]

Originally designed as a standalone graph query system, but in real world scenarios, disaster recovery capabilities is required.

We plan to offering a deployment approach based on master-slave replication, and in order to avoid modifying the core engine, we plan to separate the fault-tolerant features, encapsulating them in a separate server called proxy_server.

The proxy_server is designed to

• Connect to server interactive workers.
• Forward the received request to the workers, and gather the responses.
• Periodically check the healthiness of workers. If a worker are down, proxy will not send request to it anymore. If ALL workers are down, the proxy will return error to client.

It seems that we can not avoid introducing new actor, handler and service definitions.

The usage of proxy_server

./bin/proxy_server -e localhost:10000,localhost:10001

TODO: Currently heartbeat will only return OK, however, we need SNAPSHOT_ID to be returned.