@Override
    public Set<K> keySet() {
        return map.keySet();
    }
Checks whether the given key can be used in mutation(e.g. {@code put()},
{@code remove()}), regardless the map is freezed or not.

@param key non-null key
@return true if the given key can be used in mutation; false otherwise
clickhouse-data/src/main/java/com/clickhouse/data/ClickHouseFreezableMap.java