Add a collection named ``collection``.

def add_collection(collection,
                   cache_size=1000,
                   cache_cls=LRUCache,
                   **cache_args):
    """Add a collection named ``collection``."""
    assert collection not in _db
    cache = cache_cls(maxsize=cache_size,
                      missing=lambda id: load_object(collection, id),
                      **cache_args)
    _db[collection] = aadict(cache=cache, indexes={})