chainfee: allow specifying min relay feerate from API source

This commit adds a new expected field, `min_relay_feerate`, in the
response body returned from the API source, allowing the API to specify
a min relay feerate to be used instead of the FeePerKwFloor.

This change is backwards compatible as for an old API source which
doesn't specify the `min_relay_feerate`, it will be interpreted as zero.

lnwallet/chainfee/rates.go

@@ -53,7 +53,7 @@ func (s SatPerKVByte) FeePerKWeight() SatPerKWeight {
 
 // String returns a human-readable string of the fee rate.
 func (s SatPerKVByte) String() string {
-	return fmt.Sprintf("%v sat/kb", int64(s))
+	return fmt.Sprintf("%v sat/kvb", int64(s))
 }
 
 // SatPerKWeight represents a fee rate in sat/kw.