Used to track blocks whose transactions were applied to the UTXO state as a part of a single ActivateBestChainStep call.
This class also tracks transactions that are removed from the mempool as conflicts (per block) and can be used to pass all those transactions through SyncTransaction.
This class assumes (and asserts) that the conflicted transactions for a given block are added via mempool callbacks prior to the BlockConnected() associated with those transactions. If any transactions are marked conflicted, it is assumed that an associated block will always be added.
This class is single-use, once you call GetBlocksConnected() you have to throw it away and make a new one.
