Add frontend locking support

[MatthewScholefield]

Sometimes it's useful to use an index in the database as a lock:

events = db['events']
items = events['items']
with events['lock']:
    new_uuid = str(uuid4())
    if len(items) > 0:
        items[-1]['next_uuid'] = new_uuid
    items.append({'data': 'abc', 'next_uuid': None})

db['events']['lock'] would just be a bool and the request would wait for the bool to become false, change it to true, and then return, waiting for a followup request to set it back to false.

Alternatively, this could be generalized to a type of request that waits for a value to change.