multi: add height-based invoice expiry

This commit adds height-based invoice expiry for hodl invoices
that have active htlcs. This allows us to cancel our intentionally
held htlcs before channels are force closed. We only add this for
hodl invoices because we expect regular invoices to automatically
be resolved.

We still keep hodl invoices in the time-based expiry queue,
because we want to expire open invoices that reach their timeout
before any htlcs are added. Since htlcs are added after the
invoice is created, we add new htlcs as they arrive in the
invoice registry. In this commit, we allow adding of duplicate
entries for an invoice to be added to the expiry queue as each
htlc arrives to keep implementation simple. Our cancellation
logic can already handle the case where an entry is already
canceled, so this is ok.

lncfg/invoices.go

@@ -0,0 +1,12 @@
+package lncfg
+
+// DefaultHoldInvoiceExpiryDelta defines the number of blocks before the expiry
+// height of a hold invoice's htlc that lnd will automatically cancel the
+// invoice to prevent the channel from force closing. This value *must* be
+// greater than DefaultIncomingBroadcastDelta to prevent force closes.
+const DefaultHoldInvoiceExpiryDelta = DefaultIncomingBroadcastDelta + 2
+
+// Invoices holds the configuration options for invoices.
+type Invoices struct {
+	HoldExpiryDelta uint32 `long:"holdexpirydelta" description:"The number of blocks before a hold invoice's htlc expires that the invoice should be canceled to prevent a force close. Force closes will not be prevented if this value is not greater than DefaultIncomingBroadcastDelta."`
+}