AsyncDataloader improvements for Rails

[rmosolgo]

The AsyncDataloader could be improved in some ways to support Rails better:

• With isolation_level = :fiber, if you call connected_to { ... } in the surrounding context (parent Fiber), then child Fibers don't respect that -- but they should in this case.
• Database connections used by Fibers aren't released automatically when the Fiber terminates. AsyncDataloader should manually release its connection when its work is finished

I think these are both good defaults and it can be accomplished with a Rails-specific module included as-needed, or something similar.