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/node/interfaces.cpp

@@ -281,6 +281,10 @@ public:
     {
         return MakeHandler(::uiInterface.ShowProgress_connect(fn));
     }
+    std::unique_ptr<Handler> handleInitWallet(InitWalletFn fn) override
+    {
+        return MakeHandler(::uiInterface.InitWallet_connect(fn));
+    }
     std::unique_ptr<Handler> handleNotifyNumConnectionsChanged(NotifyNumConnectionsChangedFn fn) override
     {
         return MakeHandler(::uiInterface.NotifyNumConnectionsChanged_connect(fn));