This change modularizes the `WalletDB` and `Manifest`. Note that the wallet db has nothing to do with account wallets and it stores node configuration, which is why it depends on the manifest code.
Currently we're passing the `Application` object around, whereby the `Application` class acts more like a service registry that gives other classes access to other services. In order to allow modularization, we should replace `Application` with a service registry class so that modules depending on `Application` for other services can be moved easily. This change adds the `ServiceRegistry` class.
