All messages must have unordered identifier (UUID) in the [$msg] collection

Currently, XDI2 will accept messages as ordered members of a collection:

```
=sender[$msg]@0$do/$get/...
```

However, it should be required that every message be an unordered member (using a UUID):

```
=sender[$msg]!:uuid:1234$do/$get/...
```

If messages are to be ordered, they should use a $ref pattern:

```
=sender[$msg]@0/$ref/=sender[$msg]!:uuid:1234
```
