adjust LogMiner batch size based on comparison with currently used batch size

Currently determining whether to increase/decrease the log mining batch size is done here: https://github.com/debezium/debezium/blob/1.9/debezium-connector-oracle/src/main/java/io/debezium/connector/oracle/logminer/LogMinerStreamingChangeEventSource.java#L586-L602

There's a comparison based on the default batch size, where I think it should involve the current batch size.

(Also it feels weird that the default batch size currently has some extra - hidden - meaning, makes it harder to tune properly.)

Already opened a PR https://github.com/debezium/debezium/pull/3411 (which I'm about to update with the issue number)