public boolean isFastSyncInProgress() {
    return pivotBlockHeaderFile.isFile();
  }
Supports persisting fast sync state to disk to enable resuming after a restart.

<p>Note that a {@link FastSyncState} with a block number selected but no pivot block header is
not stored. If we haven't yet retrieved and confirmed the actual block header we can't have
started downloading data so should pick a new pivot block when resuming. Once we have the pivot
block header we want to continue with that pivot block so the world state downloaded matches up.
ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/fastsync/FastSyncStateStorage.java