Resubmit transactions from rolled back blocks

Duncan:
> Once we get to Praos and we get frequent short forks, we'll want to take txs from blocks that got rolled back and put them back into the mempool. This is "just" a performance optimisation since wallets will re-submit anyway. So how to do this in a simple modular way? Idea...
Do it asynchronously. A thread uses the chain db chain follower api to follow the current chain. When it rolls back, query the volatile db for the blocks that were rolled back, extract the txs and try to add them to the end of the mempool.
So the chain db and mempool should not have to be aware of this at all. It's just best effort, no guarantee if we fork extremely quickly and the thread cannot keep up.