[refactor] change Workspace::m_conflicts and adjacent funcs/structs to use Txid

It's preferable to use type-safe transaction identifiers to avoid
confusing txid and wtxid. The next commit will add a reference to this
set; we use this opportunity to change it to Txid ahead of time instead
of adding new uses of uint256.

src/validation.cpp

@@ -582,7 +582,7 @@ private:
     struct Workspace {
         explicit Workspace(const CTransactionRef& ptx) : m_ptx(ptx), m_hash(ptx->GetHash()) {}
         /** Txids of mempool transactions that this transaction directly conflicts with. */
-        std::set<uint256> m_conflicts;
+        std::set<Txid> m_conflicts;
         /** Iterators to mempool entries that this transaction directly conflicts with. */
         CTxMemPool::setEntries m_iters_conflicting;
         /** Iterators to all mempool entries that would be replaced by this transaction, including