Cleanup BankForks access code in ShredFetchStage

[steviez]

Summary of Changes

- BankForks is not an optional argument, so remove dated comment
- Given that BankForks is always present, no need for special values to
  initialize variables before the loop
- Root slot can be retrieved from root bank, no need for second call to
  BankForks (which would load the underlying atomic a second time)
- Use BankForks::highest_slot() instead of .slot() on .working_bank() to
  avoid the extra clone that .working_bank() performs
- Move several operations outside of BankForks read lock scope to
  minimize lock time