Introduce retry_invoice_request_message code within timer_tick_occurred.

1. And introduce static CALL_COUNTER, to trigger the `timer_tick_occurred`
   code only every 10th call.
2. Decrease Freshness timer duration.
3. This is done so that `retry_invoice_request_messages` can be called faster
   than the rest of the code without introducing a secondary timer.

Author: shaavan

lightning-background-processor/src/lib.rs

@@ -93,7 +93,7 @@ pub struct BackgroundProcessor {
 }
 
 #[cfg(not(test))]
-const FRESHNESS_TIMER: u64 = 60;
+const FRESHNESS_TIMER: u64 = 6;
 #[cfg(test)]
 const FRESHNESS_TIMER: u64 = 1;