utxonursery: connect to time-based sweeper

Previously, nursery generated and published its own sweep txes. It
stored the sweep tx in nursery_store to prevent a new tx with a new
sweep address from being generated on restart.

In this commit, sweep generation and publication is removed from nursery
and delegated to the sweeper. Also the confirmation notification is
received from the sweeper.

test_utils.go

@@ -316,7 +316,9 @@ func createTestPeer(notifier chainntnfs.ChainNotifier,
 	}
 	bobPool.Start()
 
-	chainIO := &mockChainIO{}
+	chainIO := &mockChainIO{
+		bestHeight: fundingBroadcastHeight,
+	}
 	wallet := &lnwallet.LightningWallet{
 		WalletController: &mockWalletController{
 			rootKey:               aliceKeyPriv,