wtdb: let DeleteCommitedUpdates move session to terminal

In this commit, we adjust the DeleteCommitmentUpdate method so that it
marks a session as Terminal (if there are updates to delete) since once
we have deleted a commitment update from a session - the session is no
longer useable.

watchtower/wtdb/client_db_test.go

@@ -1002,6 +1002,44 @@ func testMarkChannelClosed(h *clientDBHarness) {
 	// Assert that we now can delete the session.
 	h.deleteSession(session1.ID, nil)
 	require.Empty(h.t, h.listClosableSessions(nil))
+
+	// We also want to test that a session can be deleted if it has been
+	// marked as terminal.
+
+	// Create session2 with MaxUpdates set to 5.
+	session2 := h.randSession(h.t, tower.ID, 5)
+	h.insertSession(session2, nil)
+
+	// Create and register channel 7.
+	chanID7 := randChannelID(h.t)
+	h.registerChan(chanID7, nil, nil)
+
+	// Add two updates for channel 7 in session 2. Ack one of them so that
+	// the mapping from this channel to this session is created but don't
+	// ack the other since we want to delete the committed update later.
+	update = randCommittedUpdateForChannel(h.t, chanID7, 1)
+	h.commitUpdate(&session2.ID, update, nil)
+	h.ackUpdate(&session2.ID, 1, 1, nil)
+
+	update = randCommittedUpdateForChannel(h.t, chanID7, 2)
+	h.commitUpdate(&session2.ID, update, nil)
+
+	// Check that attempting to delete the session will fail since it is not
+	// yet considered closable.
+	h.deleteSession(session2.ID, wtdb.ErrSessionNotClosable)
+
+	// Now delete the added committed updates. This should put the
+	// session in the terminal state after which we should be able to
+	// delete the session.
+	h.deleteCommittedUpdates(&session2.ID, nil)
+
+	// Marking channel 7 as closed will now return session 2 since it has
+	// been marked as terminal.
+	sl = h.markChannelClosed(chanID7, 1, nil)
+	require.ElementsMatch(h.t, sl, []wtdb.SessionID{session2.ID})
+
+	// We should now be able to delete the session.
+	h.deleteSession(session2.ID, nil)
 }
 
 // testAckUpdate asserts the behavior of AckUpdate.