Create new `clickhouse_driver.Client`s for each `ClickHouseChain`

[michaelosthege]

That's the recommendation to get good performance, most importantly when working with multiple threads.

See https://clickhouse-driver.readthedocs.io/en/latest/quickstart.html#async-and-multithreading

To avoid having dangling client connections, this be implemented with a disposal mechanism (see #13).