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.
Currently it is an alias to the global ::mempool and should be used as
follows.
* Node code (validation and transaction relay) can use either ::mempool
or node.mempool, whichever seems a better fit.
* RPC code should use the added convenience getter EnsureMempool, which
makes sure the mempool exists before use. This prepares the RPC code
to a future where the mempool might be disabled at runtime or compile
time.
* Test code should use m_node.mempool directly, as the mempool is always
initialized for tests.
