Fix wallet unload race condition

Currently it's possible for ReleaseWallet to delete the CWallet pointer while
it is processing BlockConnected, etc chain notifications.

To fix this, unregister from notifications earlier in UnloadWallet instead of
ReleaseWallet, and use a new RegisterSharedValidationInterface function to
prevent the CValidationInterface shared_ptr from being deleted until the last
notification is actually finished.

src/wallet/test/wallet_test_fixture.cpp

@@ -10,7 +10,6 @@ WalletTestingSetup::WalletTestingSetup(const std::string& chainName)
 {
     bool fFirstRun;
     m_wallet.LoadWallet(fFirstRun);
-    m_wallet.handleNotifications();
-
+    m_chain_notifications_handler = m_chain->handleNotifications({ &m_wallet, [](CWallet*) {} });
     m_chain_client->registerRpcs();
 }

src/wallet/test/wallet_test_fixture.h

@@ -23,6 +23,7 @@ struct WalletTestingSetup: public TestingSetup {
     std::unique_ptr<interfaces::Chain> m_chain = interfaces::MakeChain(m_node);
     std::unique_ptr<interfaces::ChainClient> m_chain_client = interfaces::MakeWalletClient(*m_chain, {});
     CWallet m_wallet;
+    std::unique_ptr<interfaces::Handler> m_chain_notifications_handler;
 };
 
 #endif // BITCOIN_WALLET_TEST_WALLET_TEST_FIXTURE_H