Use JSON partial updates

[marein]

MySQL is used as a document store in the connect four context. Currently the aggregate column gets the whole serialized game when changes are made, see here. This can be improved by partially updating the JSON column which is available since MySQL version 8.0, see here.

[marein]

Tested this a few months ago. No notable difference in performance. Would only be interesting for replication or reducing network traffic at the moment. The added code complexity doesn't justify this, as neither of the two benefits are currently bottlenecks.