Support multiple query operations in a single request/query

[eslingerbryan]

The query and operation structs need some clarifying and the execution model needs to be refactored to support resolving multiple operations in a single query. Additionally, this execution should be fully parallelized among the operations. It may make sense to introduce the concept of a data loader at this time as well. Data loaders will be implemented as GenServers keeping track of an :ets cache with LRU eviction and flexible key spaces. It's not immediately clear if resolvers should use data loaders automatically, or if data loaders should be made available for resolvers to use in the function bodies. It is possible these are not mutually exclusive.

https://spec.graphql.org/June2018/#sec-Executing-Operations