Use UUID to name temp XMLRPC files

[The current way of naming temp files using PID][Link] can be
problematic if there are concurrent uploads.

[Link]: https://github.com/wordpress-mobile/WordPressKit-iOS/blob/e7d2c04c1be6c602c2f285643f6e1e19f935a8d7/WordPressKit/WordPressOrgXMLRPCApi.swift#L257-L261

Author: crazytonyli

WordPressKit/HTTPRequestBuilder.swift

@@ -215,7 +215,7 @@ final class HTTPRequestBuilder {
         request.setValue("text/xml", forHTTPHeaderField: "Content-Type")
         let encoder = WPXMLRPCEncoder(method: xmlrpcRequest.method, andParameters: xmlrpcRequest.parameters)
         if forceWriteToFile {
-            let fileName = "\(ProcessInfo.processInfo.globallyUniqueString)_file.xmlrpc"
+            let fileName = "\(UUID().uuidString).xmlrpc"
             let fileURL = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(fileName)
             try encoder.encode(toFile: fileURL.path)