htlcswitch: add an always on mode to interceptable switch

Co-authored-by: Juan Pablo Civile <elementohb@gmail.com>

sample-lnd.conf

@@ -347,6 +347,9 @@
 ; used as a hop.
 ; rejecthtlc=true
 
+; If true, all HTLCs will be held until they are handled by an interceptor
+; requireinterceptor=true
+
 ; If true, will apply a randomized staggering between 0s and 30s when
 ; reconnecting to persistent peers on startup. The first 10 reconnections will be
 ; attempted instantly, regardless of the flag's value