autopilot: use updateBalance rather than tracking balance explicitly

In this commit, we modify the balanceUpdate autopilot signal to update
the balance according to what's returned to the WalletBalance callback
rather than explicitly tracking the balance. This gives the agent a
better sense of what the wallet's balance actually is.

pilot.go

@@ -224,8 +224,8 @@ func initAutoPilot(svr *server, cfg *autoPilotConfig) (*autopilot.Agent, error)
 
 		for {
 			select {
-			case txnUpdate := <-txnSubscription.ConfirmedTransactions():
-				pilot.OnBalanceChange(txnUpdate.Value)
+			case <-txnSubscription.ConfirmedTransactions():
+				pilot.OnBalanceChange()
 			case <-svr.quit:
 				return
 			}