Hook Store into online change completion

I removed finishedAlter() in #1037 as I thought the FDB Directory needs could be serviced by the new SchemaManager#finishOnlineChange() method. They can but 1) it makes things awfully subtle 2) affects actual stored data (e.g. dir.remove()), which is normally reserved for Store. Instead, add back/a new Store method that gets the ChangeSets to continue serving the previous purpose.

Also tighten up the unused StorageDescription cleanup after an ALTER as e.g. dropIndex() isn't the correct thing when only storage needs dropped.

Lastly, a nasty typo (createDrop vs createAdd) in TableChangeValidator. Simple fix and test.