Add multi-host support

[okarpov]

It would be great to have multi-host support (as some other language drivers do), for example:

Host=111.222.33.44:8123,44.55.66.77:8123;Database=default;User=default;Password=1q2w3e4r;Compress=True;CommandTimeout=300;ReadWriteTimeout=300;

Which would provide automatic failover and load-balancing for inserts/select queries with ClickHouse replica configuration.