watchtower: add ListClosableSessions method

This commit adds a new ListClosableSessions method to the tower client
DB. This method will return a map of sessionIDs to block heights. The
IDs belong to sessions that are considered closable and the block
heights are the block height at which the last associated channel for
the session was closed in.

watchtower/wtdb/client_db_test.go

@@ -207,6 +207,17 @@ func (h *clientDBHarness) markChannelClosed(id lnwire.ChannelID,
 	return closableSessions
 }
 
+func (h *clientDBHarness) listClosableSessions(
+	expErr error) map[wtdb.SessionID]uint32 {
+
+	h.t.Helper()
+
+	closableSessions, err := h.db.ListClosableSessions()
+	require.ErrorIs(h.t, err, expErr)
+
+	return closableSessions
+}
+
 // newTower is a helper function that creates a new tower with a randomly
 // generated public key and inserts it into the client DB.
 func (h *clientDBHarness) newTower() *wtdb.Tower {
@@ -711,11 +722,16 @@ func testMarkChannelClosed(h *clientDBHarness) {
 	// since it has an update for channel 6 which is still open.
 	sl = h.markChannelClosed(chanID5, 1, nil)
 	require.Empty(h.t, sl)
+	require.Empty(h.t, h.listClosableSessions(nil))
 
 	// Finally, if we close channel 6, session 1 _should_ be in the closable
 	// list.
-	sl = h.markChannelClosed(chanID6, 1, nil)
+	sl = h.markChannelClosed(chanID6, 100, nil)
 	require.ElementsMatch(h.t, sl, []wtdb.SessionID{session1.ID})
+	slMap := h.listClosableSessions(nil)
+	require.InDeltaMapValues(h.t, slMap, map[wtdb.SessionID]uint32{
+		session1.ID: 100,
+	}, 0)
 }
 
 // testAckUpdate asserts the behavior of AckUpdate.