Add subscription support to GraphQL (execution and resolvers)

[Jannis]

This includes:

1. Adding an execute_subscription function that implements the subscribe algorithm from the GraphQL spec.
2. Adding stream resolution to Resolver and StoreResolver.

[Jannis]

Some more requirements for this:

• When executing a subscription, identify all entity types involved in the subscription query. Use those for the store subscription.
• When executing a subscription, get the subgraph ID from the subscription / query object and use it for the store subscription.
• Write the following test (similar to the store subscription test):
  • run a subscription using the query runner
  • the subscription query should involve at least two entity types
  • use a mock store that simulates entity change events for all of those entity types
  • verify that two correct query results are written to the subscription result stream.