Add WalletClient interface so node interface is cleaner and don't need
wallet-specific methods.
The new NodeContext::wallet_client pointer will also be needed to eliminate
global wallet variables like ::vpwallets, because createWallet(), loadWallet(),
getWallets(), etc methods called by the GUI need a way to get a reference to
the list of open wallets if it is no longer a global variable.
Also tweaks splash screen registration for load wallet events to be delayed
until after wallet client is created.
Route calls during node initialization and shutdown that would happen between a
node process and wallet processes through the serializable `Chain::Client`
interface, rather than `WalletInitInterface` which is now simpler and only
deals with early initialization and parameter interaction.
This commit mostly does not change behavior. The only change is that the
"Wallet disabled!" and "No wallet support compiled in!" messages are now logged
earlier during startup.
