Get a **query** message.

def query(options, collection_name, num_to_skip,
          num_to_return, query, field_selector, opts, check_keys=False):
    """Get a **query** message.
    """
    data = struct.pack("<I", options)
    data += bson._make_c_string(collection_name)
    data += struct.pack("<i", num_to_skip)
    data += struct.pack("<i", num_to_return)
    encoded = bson.BSON.encode(query, check_keys, opts)
    data += encoded
    max_bson_size = len(encoded)
    if field_selector is not None:
        encoded = bson.BSON.encode(field_selector, False, opts)
        data += encoded
        max_bson_size = max(len(encoded), max_bson_size)
    (request_id, query_message) = __pack_message(2004, data)
    return (request_id, query_message, max_bson_size)