Work-around transaction unload not informing MetaSema.

When a file is unloaded its entry should be removed from MetaSema::m_WaterMarks.
At the moment the corresponding callback is not implemented.

Instead, we also record the relation <Transaction*,Entry*>, so that upon
MetaSema requesting an unload, we can clean-out the corresponding data
from m_WaterMarks.

In addition, to catch the case where the target transaction (unloadPoint)
is not longer present we issue an error and do not unload anything.