txgraph: initialize Ref in AddTransaction (preparation)

Instead of returning a TxGraph::Ref from TxGraph::AddTransaction(),
pass in a TxGraph::Ref& which is updated to refer to the new transaction
in that graph.

This cleans up the usage somewhat, avoiding the need for dummy Refs in
CTxMemPoolEntry constructor calls, but the motivation is that a future
commit will allow a callback to passed to MakeTxGraph to define a
fallback order on the transaction objects. This does not work when a
Ref is created separately from the CTxMemPoolEntry it ends up living in,
as passing the newly-created Ref to the callback would be UB before it's
emplaced in its final CTxMemPoolEntry.

src/txgraph.h

@@ -67,12 +67,14 @@ public:
 
     /** Virtual destructor, so inheriting is safe. */
     virtual ~TxGraph() = default;
-    /** Construct a new transaction with the specified feerate, and return a Ref to it.
+    /** Initialize arg (which must be an empty Ref) to refer to a new transaction in this graph
+     *  with the specified feerate.
+     *
      *  If a staging graph exists, the new transaction is only created there. feerate.size must be
-     *  strictly positive. In all further calls, only Refs created by AddTransaction() are allowed
+     *  strictly positive. In all further calls, only Refs passed to AddTransaction() are allowed
      *  to be passed to this TxGraph object (or empty Ref objects). Ref objects may outlive the
-     *  TxGraph they were created for. */
-    [[nodiscard]] virtual Ref AddTransaction(const FeePerWeight& feerate) noexcept = 0;
+     *  TxGraph they were added to. */
+    virtual void AddTransaction(Ref& arg, const FeePerWeight& feerate) noexcept = 0;
     /** Remove the specified transaction. If a staging graph exists, the removal only happens
      *  there. This is a no-op if the transaction was already removed.
      *
@@ -235,8 +237,7 @@ public:
         /** Index into the Graph's m_entries. Only used if m_graph != nullptr. */
         GraphIndex m_index = GraphIndex(-1);
     public:
-        /** Construct an empty Ref. Non-empty Refs can only be created using
-         *  TxGraph::AddTransaction. */
+        /** Construct an empty Ref. It can be initialized through TxGraph::AddTransaction. */
         Ref() noexcept = default;
         /** Destroy this Ref. If it is not empty, the corresponding transaction is removed (in both
          *  main and staging, if it exists). */