Disallow connector to be registered if another one exists with the same database.server.name exists

[vaibhav-yb]

Since the database.server.name is the logical name for the connector, it has to be unique across all the deployed connectors.

The problem currently from the user's perspective is that they may end up deploying connectors with same database.server.name - to tackle this, see if the mechanism can be implemented where we disallow the connector deployment if another connector with the same database.server.name exists in the Kafka Connect environment.