multiprocess: Delay wallet client construction

Delay wallet client construction until after logging, thread and other
init for two reasons:

- More responsive multiprocess GUI startup. When bitcoin-gui is started
  this moves the call from bitcoin-gui to bitcoin-node that spawns
  bitcoin-wallet off of the GUI event thread and onto the background GUI
  init executor thread.

- Avoids feature_logging.py test failures with bitcoin-node by making
  bitcoin-wallet logging start after bitcoin-node logging starts,
  because the tests are not written to handle the bitcoin-wallet logging
  init code running first.

This partially reverts commit b266b3e0bf,
moving wallet client creation back to the place it was located before.

src/interfaces/node.h

@@ -197,6 +197,10 @@ public:
     using ShowProgressFn = std::function<void(const std::string& title, int progress, bool resume_possible)>;
     virtual std::unique_ptr<Handler> handleShowProgress(ShowProgressFn fn) = 0;
 
+    //! Register handler for wallet client constructed messages.
+    using InitWalletFn = std::function<void()>;
+    virtual std::unique_ptr<Handler> handleInitWallet(InitWalletFn fn) = 0;
+
     //! Register handler for number of connections changed messages.
     using NotifyNumConnectionsChangedFn = std::function<void(int new_num_connections)>;
     virtual std::unique_ptr<Handler> handleNotifyNumConnectionsChanged(NotifyNumConnectionsChangedFn fn) = 0;