open fun truncate(path: String, len: Long, handler: Handler<AsyncResult<Void>>): FileSystem
Truncate the file represented by path to length len in bytes, asynchronously.
The operation will fail if the file does not exist or len is less than zero.
len - the length to truncate it to
handler - the handler that will be called on completion
Return
a reference to this, so the API can be used fluently