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 b266b3e0bf29d0f3d5deaeec62d57c5025b35525,
moving wallet client creation back to the place it was located before.