multi: extract wallet initialization

With this commit we extract the wallet creation/unlocking and
initialization completely out of the main function. This will allow us
to use custom implementations in the future.

cmd/lnd/main.go

@@ -31,7 +31,7 @@ func main() {
 		// Help was requested, exit normally.
 		os.Exit(0)
 	}
-	implCfg := loadedConfig.ImplementationConfig()
+	implCfg := loadedConfig.ImplementationConfig(shutdownInterceptor)
 
 	// Call the "real" main in a nested manner so the defers will properly
 	// be executed in the case of a graceful shutdown.