multi: add new config usestatusinitiated for the new payment status

This commit adds a new config value to signal that the user understands
the new payment status `StatusInitiated`.

sample-lnd.conf

@@ -1201,6 +1201,10 @@
 ; failures in channels. 
 ; routerrpc.bimodal.decaytime=168h
 
+; If set, the router will send `Payment_INITIATED` for new payments, otherwise
+; `Payment_In_FLIGHT` will be sent for compatibility concerns.
+; routerrpc.usestatusinitiated=false
+
 
 [workers]