fatxpool: retracted transaction shall be prioritorized over the mempool

[michalkucharczyk]

Currently the transactions from retracted block are submitted to the block, and later (if view is at its limits) could be dropped by the transaction from the mempool.

It should be guaranteed that retracted transactions are not dropped and are prioritized over the txs from the mempool.

Retracted transactions shall also be added to the mempool (or other buffer keeping retracted transactions). Adding to mempool may require adding new flag.