Feature: Implement WebSocket durable state upon Cloudflare workers

[williamlsh]

Since Cloudflare workers are actually stateless, creating a websocket global state is impossible. though its runtime provides Durable Objects, but this feature is not within free plan.

Upstash provides a service, making REST API over Redis possible, and it has a free plan. It should suffice this need.

References:

• https://upstash.com/blog/redis-cloudflare-workers
• https://docs.upstash.com/redis/features/restapi

[williamlsh]

Currently, Upstash Redis doesn't fully support Pub/Sub, so a redis client can't subscribe to a channel.

A possible workaround is to use a List instead. To mimic subscribe, Poping a list continually in a loop with a short sleeping time.

Reference:

• https://docs.upstash.com/redis/features/restapi#rest---redis-api-compatibility