Purgable tombstones can cause false positives in repaired data tracking

Calculating the repaired data digest on the read path (for the purposes of detecting mismatches in the repaired data across replicas) is done before purging any tombstones due to gcgs or last repaired time. This causes false positives when repaired sstables include GC-able tombstones on some replicas but not others.

Validation compactions do purge tombstones so it's perfectly possible for sstables to mismatch in this way without causing any streaming during repair.