make active defrag test more stable

on slower machines, the active defrag test tended to fail.
although the fragmentation ratio was below the treshold, the defragger was
still in the middle of a scan cycle.

this commit changes:
- the defragger uses the current fragmentation state, rather than the cache one
  that is updated by server cron every 100ms. this actually fixes a bug of
  starting one excess scan cycle
- the test lets the defragger use more CPU cycles, in hope that the defrag
  will be faster, but also give it more time before we give up.

src/defrag.c

@@ -867,9 +867,8 @@ void defragDictBucketCallback(void *privdata, dictEntry **bucketref) {
  * or not, a false detection can cause the defragmenter to waste a lot of CPU
  * without the possibility of getting any results. */
 float getAllocatorFragmentation(size_t *out_frag_bytes) {
-    size_t resident = server.cron_malloc_stats.allocator_resident;
-    size_t active = server.cron_malloc_stats.allocator_active;
-    size_t allocated = server.cron_malloc_stats.allocator_allocated;
+    size_t resident, active, allocated;
+    zmalloc_get_allocator_info(&allocated, &active, &resident);
     float frag_pct = ((float)active / allocated)*100 - 100;
     size_t frag_bytes = active - allocated;
     float rss_pct = ((float)resident / allocated)*100 - 100;