multi: introduce config-level DustThreshold for defining threshold

sample-lnd.conf

@@ -365,6 +365,11 @@
 ; propagation (default: 10)
 ; max-commit-fee-rate-anchors=5
 
+; A threshold defining the maximum amount of dust a given channel can have
+; after which forwarding and sending dust HTLC's to and from the channel will
+; fail. This amount is expressed in satoshis. (default: 500000)
+; dust-threshold=1000000
+
 ; If true, lnd will abort committing a migration if it would otherwise have been
 ; successful. This leaves the database unmodified, and still compatible with the
 ; previously active version of lnd.