[wallet] Construct CWalletTx objects in CommitTransaction

Construct CWalletTx objects in CWallet::CommitTransaction, instead of having
callers do it. This ensures CWalletTx objects are constructed in a uniform way
and all fields are set.

This also makes it possible to avoid confusing and wasteful CWalletTx copies in
https://github.com/bitcoin/bitcoin/pull/9381

There is no change in behavior.

src/qt/walletmodeltransaction.h

@@ -20,11 +20,10 @@ class WalletModelTransaction
 {
 public:
     explicit WalletModelTransaction(const QList<SendCoinsRecipient> &recipients);
-    ~WalletModelTransaction();
 
     QList<SendCoinsRecipient> getRecipients() const;
 
-    CWalletTx *getTransaction() const;
+    CTransactionRef& getTransaction();
     unsigned int getTransactionSize();
 
     void setTransactionFee(const CAmount& newFee);
@@ -39,7 +38,7 @@ public:
 
 private:
     QList<SendCoinsRecipient> recipients;
-    CWalletTx *walletTransaction;
+    CTransactionRef walletTransaction;
     std::unique_ptr<CReserveKey> keyChange;
     CAmount fee;
 };