Fix stream polling casting of cursor as date string on sqlite to fix bug originally reported in #961

While it seems that the right fix would be replacing: 

```python
SQLITE_SORT_CLAUSES.update({
    'date': 'sentry_groupedmessage.last_seen',
    'new': 'sentry_groupedmessage.first_seen',
})
```
with 
```python
SQLITE_SORT_CLAUSES.update({
    'date': "strftime('%s', sentry_groupedmessage.last_seen)",
    'new': "strftime('%s', sentry_groupedmessage.first_seen)",
})
```
, that throws a mysterious error I can't seem to trace back. What's up with error reporting? I get a stack, but no root cause. 

So this works around it by parsing the date string as an integer in the javascript when the poll occurs.