Support for Subscriptions/Live Queries

[dzhelezov]

Many customers has requested supports for subscription support so that they don't have to constantly poll the graphql server for updates.

The most convenient solution is probably something similar to live queries supported by PostGraphile and a separate lib(e.g. this. Hasura also supports subscriptions

Let's break down this task into multiple phases:

• Research the existing solutions regarding the convenience and performance trade-offs
• Develop a spec for additional directives to be added to OpenReader
• Implement as a core feature