joins support

[antonio-antuan]

OK, here is a join support for standard sqla-core (issue) and some kind of joins-api refactor.

Example is here: tests.sql.test_schema.SchemaTestCase#test_reflect.

The magic is here: clickhouse_sqlalchemy.drivers.base.ClickHouseDialect#reflecttable.
I replace table bounded with metadata by the table with another class.

The place that can make you sad is clickhouse_sqlalchemy.orm.query.Query#join. But I hope it is not: can't invent another way to make this possible: there are a lot of work inside of sqlalchemy.orm.query.Query#join method which should be done, but in the and another class must be used.

[xzkostyan]

Very sorry for late review.

[hhell]

I replace table bounded with metadata by the table with another class

What exactly is the point and is there a test that should fail without the magic?

(currently, test_reflect fails for me in either case)

[hhell]

What exactly is the point and is there a test that should fail without the magic?

After a bit more digging:

• Tests failed because the table was created in database default and was not found in the database test
• The magic is necessary for metadata.reflect to make a clickhouse_sqlalchemy.sql.Table instance instead of sqlalchemy.sql.schema.Table, for the extra parameters support in join
• The magic was breaking the sqlalchemy.sql.schema.Table(..., autoload=True) use case (the returned table had empty schema)

I put an improvement (support and test of both use cases) into a commit in #62