Enables NIO-based Concurrent Execution

[NeedleInAJayStack]

The existing concurrent executor was a mix of DispatchQueue.async calls and NIO futures, and was only used on an opt-in basis.

This makes the following changes:

1. Adds a fully NIO-based concurrent executor
2. Makes it the default executor used on query and subscription operations
3. Deprecates APIs that allow specifying particular executors, as well as the DispatchQueue-based executor.

This should better parallelize GraphQL queries by default, resulting in improved performance.

Changes to default values are typically considered major, however this seems to be an exception. The GraphQL spec states the resolvers of queries must be side-effect free and idempotent, the execution order must not affect the result, which means that this can be considered a minor based on the resolver idempotency agreement.

The reasoning for the deprecation is that the JavaScript reference implementation does not expose executor implementations to the caller, instead always executing query and subscription concurrently and mutation serially. Furthermore, Graphiti does not allow customization of executors, implying that it is a rarely used feature.

[paulofaria]

Is this good for review?