[RFC] Support distributed flow control

[sczyh30]

Issue Description

Type: feature request

Describe what feature you want

Currently Sentinel only supports local flow control. It's necessary to support distributed flow control for service clusters, so that we can:

• enable global flow control in API Gateway (e.g. Netflix Zuul)
• provide flow control ability for service mesh

This is the umbrella issue for Sentinel cluster flow control.

Related tasks

• Design cluster token client interface
• Design cluster token server interface (should also support embedded mode)
• Provide default client and server implementation
• Integrate with Envoy global rate limiting gRPC service (Implement Envoy global rate limiting gRPC service with Sentinel cluster flow control #666)
• Integrate with Istio Mixer quota management service ([Feature] Istio v1 Mixer quota management service integration #1234)
• Istio native integration (leverage Sentinel Go)
• Distributed concurrency control

[sczyh30]

Envoy global rate limiting has been supported in 1.7.0 (via #1139)