`ClickHouseChain` batch inserting assumes that all calls to `.append()` pass similar dicts as the first call

[michaelosthege]

The batch inserting mechanism creates the insert query based on the composition of draw and stats dicts of the very first call to .append().

Later on it assumes that all further calls have the identical composition of dictionaries.

ClickHouse does support sparse insert queries, but the batch inserting mechanism will have to be adapted to check if the composition changed, and if needed commit the queue and create a new query.