Delete expired invoices after one pending check (#531)

* delete expired invoices regularly and after pending check

* fix mistake

* delete expired invoices only once

lnbits/tasks.py

@@ -82,8 +82,6 @@ async def invoice_listener():
 
 
 async def check_pending_payments():
-    await delete_expired_invoices()
-
     outgoing = True
     incoming = True
 
@@ -98,6 +96,9 @@ async def check_pending_payments():
         ):
             await payment.check_pending()
 
+        # we delete expired invoices once upon the first pending check
+        if incoming:
+            await delete_expired_invoices()
         # after the first check we will only check outgoing, not incoming
         # that will be handled by the global invoice listeners, hopefully
         incoming = False