Disallow certain changes for realtime table config

Once a table is created, some aspects of realtime table cannot be changed.

For example in case of Kafka, the topic should not be changed to a different one. If the different topic has a different number of partitions, then the partition allocation algorithm in the controller get all messed up (since it does not expect number of partitions to decrease).

Start by tracing how realtime table configuration changes reach the controller, focusing on the Kafka topic and partition assumptions described in the issue. Determine which changes must be rejected after table creation and verify that invalid topic changes cannot disrupt partition allocation; the issue names no files or tests.

Written by the indexing model from the issue text.