Retry pending transactions if they are not included in the winning block

[mitar]

If a node was mining on a set of pending transactions and its block has not won, what to do if there are any transactions which were not included in the winning block. Because we allow same transactions to appear in multiple blocks, and because we do not want to keep in memory all transactions to check if there are duplicates, it seems tricky what exactly to do.

One option would be to find a common ancestor in chains of both blocks and see which transactions are in losing chain, but not in winning, but only from that common ancestor up to the newest blocks. And then reinclude them into pending transactions.

The question is also how should that happen. By reincluding locally only by a node, or by full retransmission over the whole network.