Remove replica_set connection mode

With 2.6 on the horizon it is time to sunset this problematic mode of operation.

When replica_set connection mode is used, Debezium MongoDB connector will connect to each shard individually – circumventing Mongo router (mongos). This is something explicitly discouraged by MongoDB (or rather the docs states that clients must connect through the router) and causes various issues with signalling, incremental snapshot support and overall complexity of our codebase.

Consequence:
This mode was the only means of using multiple KC tasks with MongoDB connector as it was possible to process each shard in a separate task (provided sufficient tasks.max configuration). WIth this removal we are restricting the connector to single task operation. However a future reintroduction of multitask support is possible – based on either database or collection scoped change streams.