Manage File Expiration Rules

Some files are meant to be updated from time to time. For example sbt-update plugin uses maven-metadata.xml to query the newest versions of artifacts. In this case we need to expire maven-metadata.xml files after downloaded, here comes the expire rules：

• pattern：a regex, the pattern of the file url that needs expiration
• duration: files get expired after this time. Use format that scala.concurrent.duration.Duration.apply accepts，e.g. 1 day