Support automatical JOINs

[jstaffans]

Basically each SolvableNode knows which its child fields are and can infer JOIN relations (assuming that the datastore is a relational database):

post(id: 1) {
  author {
    name
  }
} 

This might not be doable in a completely automatic fashion, but we can at least improve the support for this case by providing some kind of solution context that is accessible by solve functions.