Get realtime updates with Cloud Firestore with asyncio

[Podolyan98]

Hi. How can I run this example through asyncio? The error I got on startup:

name col_snapshot is not defined

sample:

    async def on_snapshot(col_snapshot, changes, read_time):
        for change in changes:
            if change.type.name == 'ADDED':
                response = f'New document: {change.document.id}'
                logger.debug(response)
            elif change.type.name == 'MODIFIED':
                response = f'Modified document: {change.document.id}'
                logger.debug(response)
            elif change.type.name == 'REMOVED':
                response = f'Removed document: {change.document.id}'
                logger.debug(response)
                delete_done.set()

    col_query = db.collection(u'cities')
    query_watch = col_query.on_snapshot(await on_snapshot(col_snapshot, changes, read_time))

[crwilcox]

Deduping this with googleapis/python-firestore#131.

Watch is currently not supported on the async surface as it is a background thread today and wasn't as obvious a benefit to adding async.