Various changes to SAI index validation

- Fix checksum calculation in IncrementalChecksumSequentialWriter
- Checksum per-SSTable and per-column components after streaming
- Avoid validating indexes when full rebuild is requested

patch by Piotr Kołaczkowski; reviewed by Caleb Rackliffe and Andres de la Peña for CASSANDRA-18490

Co-authored-by: Piotr Kołaczkowski <pkolaczk@gmail.com>
Co-authored-by: Caleb Rackliffe <calebrackliffe@gmail.com>
13 files changed