Reduce DB round-trips for LOB handling

Raised by taregger in https://github.com/debezium/debezium/pull/2347#pullrequestreview-666205440:

This basically sends every LOB (or LOB chunk) again to the db server and reads back the converted value. So in the end each LOB gets transferred between database and connector 3 times. This is potentially a huge amount of data sent over the wire as we are dealing with LOBs here. Is there a reason why oracle.sql.RAW#hexString2Bytes was not used here?