Support tx poh recording in unified scheduler (#4150)

* Support tx poh recording in unified scheduler

* Make transaction_indexes allocation conditional

* Replace Option<Option<_>> with saner type

* Explain the odd transaction_status_sender.is_some()

* Explain about Option-ed closures

* Rename CommitFailed => CommitCancelled

* Fix typos

* Document and simplify pre_commit_callback handling

* Clean up pre_commit_callback wrapping code

* Extend pre_commit_callback for existing bailouts

* Clean up index population

* Feeeze-lock block-producing unified scheduler bank

ledger/benches/blockstore_processor.rs

@@ -162,6 +162,7 @@ fn bench_execute_batch(
                 &mut timing,
                 None,
                 &prioritization_fee_cache,
+                None::<fn(&_) -> _>,
             );
         }
     });