Avoid needless balance calculations from ticket autobuyer

When the ticketbuyer.balancetomaintainabsolute flag is unset/zero,
there is no reason that the account balance needs to be calculated.
On busier wallets, avoid this results in a significant performance
improvement.

This flag is already broken for a different reason (change can land in
other accounts, causing the account balance to drop below this
setting) but there are still some people using the flag despite this,
and compensating by setting it higher than they need.

ticketbuyer/tb.go

@@ -254,36 +254,42 @@ func (tb *TB) buy(ctx context.Context, passphrase []byte, tip *wire.BlockHeader,
 	splitAccount := cfg.TicketSplitAccount
 	changeAccount := cfg.ChangeAccount
 
-	// Determine how many tickets to buy
-	bal, err := w.AccountBalance(ctx, account, minconf)
-	if err != nil {
-		return err
-	}
-	spendable := bal.Spendable
-
-	if spendable < maintain {
-		log.Debugf("Skipping purchase: low available balance")
-		return nil
-	}
-	spendable -= maintain
 	sdiff, err := w.NextStakeDifficultyAfterHeader(ctx, tip)
 	if err != nil {
 		return err
 	}
-	buy := int(spendable / sdiff)
-	if buy == 0 {
-		log.Debugf("Skipping purchase: low available balance")
-		return nil
-	}
-	max := int(w.ChainParams().MaxFreshStakePerBlock)
-	if buy > max {
-		buy = max
+
+	// Determine how many tickets to buy
+	var buy int
+	if maintain != 0 {
+		bal, err := w.AccountBalance(ctx, account, minconf)
+		if err != nil {
+			return err
+		}
+		spendable := bal.Spendable
+		if spendable < maintain {
+			log.Debugf("Skipping purchase: low available balance")
+			return nil
+		}
+		spendable -= maintain
+		buy = int(spendable / sdiff)
+		if buy == 0 {
+			log.Debugf("Skipping purchase: low available balance")
+			return nil
+		}
+		max := int(w.ChainParams().MaxFreshStakePerBlock)
+		if buy > max {
+			buy = max
+		}
+	} else {
+		buy = int(w.ChainParams().MaxFreshStakePerBlock)
 	}
 	if limit == 0 && csppServer != "" {
 		buy = 1
 	} else if limit > 0 && buy > limit {
 		buy = limit
 	}
+
 	purchaseTicketReq := &wallet.PurchaseTicketsRequest{
 		Count:         buy,
 		SourceAccount: account,