readyChan/unlockChan only guarantee lnd's gRPC listener socket is bound,
not that lnd's RPC interceptor has left WAITING_TO_START. Callers that
poll litd's status (itest's WaitForLNDWalletReady, litcli status, the UI)
could observe "Wallet Ready" and then immediately hit a "waiting to
start, RPC services not available" error, which was the root cause of a
flake in TestLightningTerminal/.../terminal_stateless_init_mode (CI run
29286599979, PR #1322). Poll lnd's StateService, which is exempt from
both the macaroon and RPC-readiness checks, until it reports leaving
WAITING_TO_START before advertising the wallet as ready.
