Importer should build SSTable indexes successfully before making new SSTables readable

Importing SSTables w/ already built SAI components is pretty unproblematic. However, the case where we import SSTables without accompanying index components can be problematic in two ways, given the indexes are built after the SSTables themselves are added to the tracker:

1.) The new SSTables can be seen for reads on the base table before their attached indexes are built and ready to query in the index View.

2.) If a failure occurs in an index build, the import just leaves the index in a compromised state, and even though a failure to import will be reported to the user, the SSTables will be readable for queries on the base table.

If the index implementation allows it (and SAI does), SSTableImporter should make sure SSTable indexes are built before making SSTables visible as part of the base table. If an index build fails, the import should fail and it should appear to ongoing reads like it never happened, including the index continuing to be queryable.