2. Added attribute server_version to Database class
3. Changed Engine.create_table_sql(), Engine.drop_table_sql(), Model.create_table_sql(), Model.drop_table_sql() parameter to db from db_name
1) If database name contained some signes (+, -) create merge table failed.
2) When merge table was created and set_database was not previously called, "currentDatabase()" was also used, which could lead to errors on multiple databases.
1) Divided readonly and system flags of Field model. Readonly flag only restricts insert operations, while system flag restricts also create and drop table operations
2) Added Merge engine and tests for it
3) Added docs for Merge engine
4) Added opportunity to make Field readonly. This is useful for "virtual" columns (https://clickhouse.yandex/docs/en/single/index.html#virtual-columns)
