Added configuration to allow support for async operations

[opensaucerer]

A new field called UseAsync should be added to the BridgeConfig struct with a bool value.
With this inclusion requires also the addition of quit and done channels to the UploadedFile struct returned during any call to the UploadFile function of any rainbow bridge.
These channels allow Bifrost to signal the completion of async operations while also allowing users to signal the termination of async operations, if necessary.