wes._queue_event: cleanup data inside event multiplexors

For example event multiplexors is "wes4store" event where
all changes in models are collected in `event.json.data` array
until this event is pending. Thus many changes will be delivered
by one app-level event.
```js
    event = {
        "ev": "wes4store",
        "json": {
            "store": "lftpd",
            "data": [
                {
                    "id": "LOOPBACK",
                    "sts": "runs pid: 5444\n2014-10-28T21:34:27.493Z\n"
                },
                {
                    "id": "LOOPBACK",
                    "sts": "r:==LOOPBACK==== ok ===========\n"
                }
            ]
        }
    }
```
After such event was sent/handled do cleanup of `event.json.data`
array because in subapi it is a global event collecting variable
not new one next time:
```js
    local = {
        wes:{// `wes` event object with dynamic `data` field
            store:'lftpd',
            data:[ ]
        }
    }
```

Signed-off-by: Oleg Verych <olecom@gmail.com>