Non-memory cache and close context

[eaglemoor]

I found 2 problems in data loader.

1. We can't use non-memory cache, because cache save lambda function.
2. If user close context, batchFunc return error, and this error will be used in cache for all batch keys.

I fix it with DataCache interface and detachContext. PR https://github.com/graph-gophers/dataloader/pull/104