keep updating aux::time_now() while there are announces

This fixes an infinite loop when an announce during shutown fails and
aux::time_now() has fallen behind clock_type::now() by more than the
delay calculated in announce_entry::failed(). If this happens,
announce_entry::can_announce() can forever return true when called from
torrent::announce_with_tracker() because that function uses
clock_type::now().

Also change torrent::announce_with_tracker() to use aux::time_now() just
for consistency with the reset of the announce related functions.

src/session_impl.cpp

@@ -3052,10 +3052,13 @@ retry:
 
 		// we have to keep ticking the utp socket manager
 		// until they're all closed
+		// we also have to keep updating the aux time while
+		// there are outstanding announces
 		if (m_abort)
 		{
 			if (m_utp_socket_manager.num_sockets() == 0
-				&& m_undead_peers.empty())
+				&& m_undead_peers.empty()
+				&& m_tracker_manager.empty())
 				return;
 #if defined TORRENT_ASIO_DEBUGGING
 			fprintf(stderr, "uTP sockets left: %d undead-peers left: %d\n"

src/torrent.cpp

@@ -3206,7 +3206,7 @@ namespace {
 		req.num_want = (req.event == tracker_request::stopped)
 			? 0 : settings().get_int(settings_pack::num_want);
 
-		time_point now = clock_type::now();
+		time_point now = aux::time_now();
 
 		// the tier is kept as INT_MAX until we find the first
 		// tracker that works, then it's set to that tracker's