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.
