Fix integer overflow in range_for_up_to_consensus_round
Prevent integer underflow in transaction_deferral_within_limit

Add a debug_assert! to enforce the invariant during object creation; and adjust future_round to be at least deferred_from_round in release builds to maintain the invariant without panicking in new_for_consensus_round
Add debug_assert!(future_round >= deferred_from_round); and use saturating_sub in transaction_deferral_within_limit.

Revert the change in range_for_up_to_consensus_round to keep checked_add(1).unwrap()

import debug_fatal