Currently inBlock is a std::set<txiter, CompareIteratorByHash>, which depends on having
a txiter (indexed_transaction_set::nth_index<0>::type::iterator). In future changes to
package handling, we want to avoid being dependent on mempool data structures of mapTx.

This commit is a step in that direction. We convert std::set<txiter, CompareIteratorByHash>
to an std::unordered_set<TxId, SaltedTxidHasher>. Which allows us to tell if a TxId has
already been added to the block template.

Depends on D2864