Add ray serve's logging context manager

[jiaodong]

Why are these changes needed?

Current logger is passed into uvicorn's logger which logs with INFO level and a bit spammy. Therefore we can wrap the block for startup / shutdown with slightly higher level within a context manager.

Related issue number

Closes #15872

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

========= Use the following local FastAPI wrapper code to repo and verify fixes ==============

import requests
from fastapi import FastAPI

import ray
from ray import serve

serve.start()
app = FastAPI()


@serve.deployment(route_prefix="/hello")
@serve.ingress(app)
class MyFastAPIDeployment:
    @app.get("/")
    def root(self):
        return "Hello, world!"

    @app.post("/{subpath}")
    def root(self, subpath: str):
        return f"Hello from {subpath}!"

MyFastAPIDeployment.deploy()

=========== Before ==============

❯ python logging_fix_stateful.py
2021-06-15 18:16:56,958	INFO services.py:1315 -- View the Ray dashboard at http://127.0.0.1:8265
(pid=87438) 2021-06-15 18:16:58,764	INFO http_state.py:72 -- Starting HTTP proxy with name 'Rhmwjp:SERVE_CONTROLLER_ACTOR:SERVE_PROXY_ACTOR-node:192.168.86.151-0' on node 'node:192.168.86.151-0' listening on '127.0.0.1:8000'
2021-06-15 18:16:58,887	INFO api.py:713 -- Started Serve instance in namespace 'serve'.
2021-06-15 18:16:58,902	INFO api.py:415 -- Updating deployment 'MyFastAPIDeployment'.
(pid=87436) INFO:     Started server process [87436]
(pid=87438) 2021-06-15 18:16:58,975	INFO backend_state.py:773 -- Adding 1 replicas to backend 'MyFastAPIDeployment'.
(pid=87438) 2021-06-15 18:16:58,981	INFO runtime_env.py:107 -- Using preinstalled conda environment: ray
(pid=87465) 2021-06-15 18:17:01,058	INFO on.py:45 -- Waiting for application startup.
(pid=87465) 2021-06-15 18:17:01,058	INFO on.py:59 -- Application startup complete.

============ After ==============

❯ python logging_fix_stateful.py
2021-06-15 22:10:37,274	INFO services.py:1315 -- View the Ray dashboard at http://127.0.0.1:8265
(pid=96391) 2021-06-15 22:10:39,400	INFO http_state.py:72 -- Starting HTTP proxy with name 'nsXmqR:SERVE_CONTROLLER_ACTOR:SERVE_PROXY_ACTOR-node:192.168.86.151-0' on node 'node:192.168.86.151-0' listening on '127.0.0.1:8000'
2021-06-15 22:10:39,538	INFO api.py:714 -- Started Serve instance in namespace 'serve'.
(pid=96389) INFO:     Started server process [96389]
2021-06-15 22:10:39,553	INFO api.py:416 -- Updating deployment 'MyFastAPIDeployment'.
(pid=96391) 2021-06-15 22:10:39,615	INFO backend_state.py:773 -- Adding 1 replicas to backend 'MyFastAPIDeployment'.
(pid=96391) 2021-06-15 22:10:39,621	INFO runtime_env.py:107 -- Using preinstalled conda environment: ray

[jiaodong]

oops latest commit removed json file for some reason -- will walk it through tomorrow, please only check the py files for now.