Remove dead checkpoint validation fan-out from query workers (#203)

Replace parallel FuturesUnordered execution paths in run_query_set with
a single sequential loop. The checkpoint validation fan-out branch was
dead code (ValidationCommand::Enable is never sent, so
checkpoint_validation.active was always false).

Changes:
- run_query_set: replace two parallel code paths with sequential for-loop
- run_single_query/execute_query: remove capture_checkpoint_batches and
  checkpoint_validation parameters; execute_query returns Result<()>
- QueryRunResult: remove checkpoint_batches field
- CheckpointValidationState -> IterationTracker: strip to only
  completed_iterations + status_tx (load runner iteration tracking)
- Remove CHECKPOINT_IN_FLIGHT_QUERIES static and
  record_checkpoint_in_flight_queries method
- Remove poll_commands/validate/record_outcome methods
- Clean up unused imports (FuturesUnordered, BoxFuture, FutureExt, etc)

Author: phillipleblanc