account: ensure only one outstanding account reservation exists

The auctioneer only expects one outstanding account reservation to exist
per trader LSAT, so we should enforce this on the trader's side as well.

account/manager.go

@@ -140,6 +140,12 @@ type Manager struct {
 	// Finalize message for.
 	pendingBatchMtx sync.Mutex
 
+	// reservationMtx prevents a trader from attempting to have more than
+	// once active reservation at a time when creating new accounts. This is
+	// done to ensure an account picks up the correct reservation once its
+	// time to fund it.
+	reservationMtx sync.Mutex
+
 	wg   sync.WaitGroup
 	quit chan struct{}
 }
@@ -260,6 +266,12 @@ func (m *Manager) QuoteAccount(ctx context.Context, value btcutil.Amount,
 func (m *Manager) InitAccount(ctx context.Context, value btcutil.Amount,
 	expiry, bestHeight, confTarget uint32) (*Account, error) {
 
+	// We'll make sure to acquire the reservation lock throughout the
+	// account funding process to ensure we use the same reservation, as
+	// only one can be active per trader LSAT.
+	m.reservationMtx.Lock()
+	defer m.reservationMtx.Unlock()
+
 	// First, make sure we have a valid amount to create the account. We
 	// need to ask the auctioneer for the maximum as it dynamically defines
 	// that value.