Optimize heartbeat action connections for connection-bounded environments

https://gitter.im/debezium/dev?at=5ed51f13778fad0b133d58cb

In connection bounded environments, the changes introduced by https://github.com/debezium/debezium/pull/1358 causes users who use the heartbeat action query to initiate 2 connections per connector to the database, one for replication and another for the heartbeat functionality.

Taylor did ask if we could introduce a toggle that would allow reusing the same connection for both, but I'm afraid that doing that would cause the problem that Jiri was trying to fix in the pull request to potentially become problematic.

Another idea would be to introduce a configuration toggle that would close the connection used by the heartbeat action query after the update has been written to the database to keep the number of active connections aligned with behavior prior to this new feature. The default would be keep the connection open for performance reasons but allow users to accept the network/performance impact in connection bounded environments.

Another (which would rely on understanding the read-only transaction failure that Jiri encountered) would be to see if the same connection could be used if this read-only transaction problem could be avoided somehow.