Motiviation: currently the FileResolver is in the io.vertx.core.impl package and owns a Vertx instance. The only reason it needs this instance is to delete the cache dir when it is closed, because it actually needs to schedule a blocking file deletion (via the filesystem instance).

Change: decouple the FileResolver from the Vertx instance and the FileSystem implementation so it can be used without Vertx or FileResolver. The close operation is now blocking and the callers run it in an execute internal blocking operation (Vert.x close) or a thread (at JVM shutdown).