proposal: re-design startup lifecycle

[seeflood]

What would you like to be added:
Re-design startup lifecycle.

Why is this needed:
Currently the startup sequence is:

1. app start. Run a grpc server waiting for sidecar to call back
2. sidecar ready (and then call back to apps, or wait for apps to poll)
3. app ready to serve/send requests

That's complex and need sofaboot support.

[seeflood]

Solution A. Busy waiting

1. Let them start concurrently
   

2. Sidecar loops calling the ListTopicSubscriptions API until it success:
   

3. The APP should loop heath checking the sidecar until it success:
   

4. After the sidecar is ready, the app begin to

• mark itself "ready"

• publish its endpoints
  For example, mosn has api for apps to publish or subscribe RPC endpoints. See https://github.com/mosn/mosn/tree/master/pkg/upstream/servicediscovery/dubbod

• and then serve or send requests

[seeflood]

similar issue: dapr/dapr#2918

[seeflood]

Solution B. Remove circular dependencies !

Circular dependencies are EVIL and we should remove them.

• Start concurrently
  

• The app waits for the sidecar to be ready
  We can implement it in the framework (for example, in the SOFA-Boot) , or using a prestart script in K8s yaml.
  

• Remove ListTopicSubscriptions because it's evil and leads to circular dependencies :)

• Add an API in sidecar for the app to push its manifest, including the topics they want to subscribe.
  

For more details of this API, see #500 (comment)

[seeflood]

I had an trouble when implementing solution A:
All API plugins need the callback connection to do Init and gRPC server won't start until they are all inited.
So it's hard to refactor the lifecycle and loop calling back in a separate goroutine