Send btcdconnected notifications without a wallet.

The notified chain server connection state was being passed through
the wallet and then notified to the RPC server, which prevented this
notification from ever firing if a wallet didn't exist yet.  Instead,
make the RPC server register for these notifications directly from the
chain server RPC client.

I'm not happy with this notification and how it's handled in the code,
but to not break existing clients this change is being made.  Fixing
the notifiation mess and modifying existing clients to use a new
notification API will need to be done sometime later.