putAsync
open suspend override fun putAsync(key: String, creationFunction: suspend (File) -> Boolean): Deferred<File?>
Associates a new file serialized by creationFunction asynchronously with the given key.
Any unhandled exceptions inside the creation block will be thrown. Existing or under-creation files associated with key will be replaced by the new file.
creationFunction should return true if the creation was successful, and false otherwise.
Returns: a Deferred that will complete with the new file if the creation was successful, or null otherwise.