An iterator of ready transactions.
The trait extends regular
std::iter::Iterator trait and allows reporting
last-returned element as invalid.
The implementation is then allowed, for performance reasons, to change the elements returned next, by e.g. skipping elements that are known to depend on the reported transaction, which yields them invalid as well.
Report given transaction as invalid.
This might affect subsequent elements returned by the iterator, so dependent transactions are skipped for performance reasons.
A no-op implementation for an empty iterator.