lnd: make MailboxDeliveryTimeout configurable

sample-lnd.conf

@@ -1423,3 +1423,10 @@ litecoin.node=ltcd
 ; window to allow more inputs to be added and thereby lower the fee per input.
 ; sweeper.batchwindowduration=30s
 
+[htlcswitch]
+
+; The timeout value when delivering HTLCs to a channel link. Setting this value
+; too small will result in local payment failures if large number of payments
+; are sent over a short period.
+; htlcswitch.mailboxdeliverytimeout=60s
+