change session opt to arg

src/main/scala/ch/memobase/ArgParser.scala

@@ -65,10 +65,9 @@ trait ArgParser {
         .text("retains only records processed before timestamp")
         .maxOccurs(1)
         .optional(),
-      opt[String]("<session-id>")
+      arg[String]("<session-id>")
         .action((v, c) => c.copy(sessionId = v))
         .text("session id assigned to delete message")
-        .unbounded()
         .required(),
       help("help").text("prints this text")
     )