Feature/auto execute

[NeedleInAJayStack]

New features:

• It adds support for concurrent access to the DataLoaders, which is important as GraphQL resolvers fan out asynchronously. This makes it possible to use a single DataLoader across all resolvers, which gets us the largest benefit from the caching and batching capabilities.
• DataLoader instances now have an executionPeriod option (enabled by default) that schedules an automatic execution of the queue, which was requested in Schedule batch loading #7. This means the client no longer has to call dispatchQueue (now execute) manually, which was difficult to do efficiently across GraphQL resolvers. The API here is inspired by the go version.
• An example using Graphiti and DataLoader was added to the readme

Apart from that, there's some minor refactoring and renaming.

Note that this does change the public API, so it should result in a major version increment.

[paulofaria]

Brilliant work!