SqliteQueueDatabase.is_stopped() returns false even after successful stop
I guess because `self._is_stopped` is never set to true in `stop()`?

To reproduce:

```
from playhouse.sqliteq import SqliteQueueDatabase

db = SqliteQueueDatabase("reproduce.db")

was_started = db.start()
print("Database was started: %s" % was_started)
was_stopped = db.stop()
print("Database was stopped: %s" % was_stopped)
print("Database is stopped: %s" % db.is_stopped())
```
