main: Add mainloop callback event flags

Implement the ability to set flags per events which influences the set up of
the event.

This commit only adds one flag which is "need network" meaning that the event
is not enabled if tor has disabled the network or if hibernation mode.

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket25376_25762

@@ -0,0 +1,10 @@
+  o Major feature (main loop, CPU usage):
+    - Previously, tor would enable at startup all possible main loop event
+      regardless if it needed them. For instance, directory authorities
+      callbacks were fired up even for client only. We have now refactored this
+      whole interface to only enable the appropriate callbacks depending on what
+      are tor roles (client only, relay, hidden service, etc.). Furthermore,
+      these events now depend on DisableNetwork or the hibernation state in
+      order to enable them. This is a big step towards reducing client CPU usage
+      by reducing the amount of wake ups the daemon does. Closes ticket 25376
+      and 25762.