qt: allow walletpassphrase in debug console without -server

Currently it is only possible to use walletpassphrase to unlock the
wallet when bitcoin is started in server mode.

Almost everything that manipulates the wallet in the RPC console
needs the wallet to be unlocked and is thus unusable without -server.

This is pretty unintuitive to me, and I'm sure it's even more confusing
to users.

Solve this with a very minimal change: by making the GUI start a
dummy RPC thread just to handle timeouts.