Lower the delay before saving guard status to disk

"Maybe this time should be reduced, since we are considering
guard-related changes as quite important? It would be a pity to
settle on a guard node, then close the Tor client fast and lose that
information."

Closes 12485.

changes/bug12485

@@ -0,0 +1,4 @@
+  o Minor features (Guard nodes):
+    - Reduce the time delay before saving guard status to disk from 10
+      minute to 30 seconds (or from one hour to 10 minutes if
+      AvoidDiskWrites is set).  Closes ticket 12485.

src/or/entrynodes.c

@@ -1525,10 +1525,10 @@ entry_guards_parse_state(or_state_t *state, int set, char **msg)
 
 /** How long will we let a change in our guard nodes stay un-saved
  * when we are trying to avoid disk writes? */
-#define SLOW_GUARD_STATE_FLUSH_TIME 3600
+#define SLOW_GUARD_STATE_FLUSH_TIME 600
 /** How long will we let a change in our guard nodes stay un-saved
  * when we are not trying to avoid disk writes? */
-#define FAST_GUARD_STATE_FLUSH_TIME 600
+#define FAST_GUARD_STATE_FLUSH_TIME 30
 
 /** Our list of entry guards has changed, or some element of one
  * of our entry guards has changed. Write the changes to disk within