In DBZ-4276 we introduced unavailable.value.placeholder behavior and as a part of that we added some tests to deal with when LOB-support is disabled. There were some inconsistent behaviors when Xstream was being used that caused tests to fail because either the placeholder wasn't being emitted or the test made some poor assumptions about what Xstream state would be.

1. Adjust tests to document the variances between LogMiner/Xstream and why
2. Align Xstream unavailable value placeholder logic as close to LogMiner as possible
3. Fix bug in before state of LOB values when using Xstream; this should always be the placeholder since Xstream cannot provide prior state for LOB columns.
4. Schema changes for new but excluded tables could cause unexpected event dispatch.