message: keep_up: simplify sync back leap era id selection
  Simplify code for era id selection when the trusted block header is not
  in storage.
diff: @@ -670,24 +670,24 @@ impl MainReactor {
  }
  Ok(None) => {
  debug!(%parent_hash, "historical: did not find block header in storage");
  - let maybe_sync_era = if block_header.era_id() == EraId::from(0) {
  + let era_id = if block_header.era_id() == EraId::from(0) {
  // if the block is in era 0 its parent can only be in era 0
  - Some(EraId::from(0))
  + EraId::from(0)
  } else {
  // we do not have the parent header and thus don't know what era
  // the parent block is in (it could be the same era or the previous
  // era). we assume the worst case and ask
  - // for the earlier era's proof
  - block_header.era_id().predecessor()
  + // for the earlier era's proof;
  + // subtracting 1 here is safe since the case where era id is 0 is
  + // handled above
  + block_header.era_id().saturating_sub(1)
  };
  - maybe_sync_era.map_or(Ok(None), |era_id| {
  Ok(Some(SyncBackInstruction::Sync {
  parent_hash: *parent_hash,
  maybe_parent_metadata: None,
  era_id,
  }))
  - })
  }
  Err(err) => Err(err.to_string()),
  }
  