Compound Annotation’s Segment Index is stored in FossilDB, should be in TemporaryStore

[fm3]

When viewing a compound annotation with volume layers, the merged volume buckts and tracing layers are stored in memory in a TemporaryStore. However, the segment index is still stored in fossildb. This is inconsistent and is a waste of storage. It should also be stored in a TemporaryStore.

Note that this should be tackled after #7839 to avoid merge conflicts.