Adjustments for active defrag defaults and tuning

Reduce default minimum effort, so that when fragmentation is just detected,
the impact on the latency will be minor.

Reduce the default maximum effort, mainly to prevent a case were a sudden
massive deletions, won't trigger an aggressive defrag that will cause latency.

When activedefrag is disabled mid-run, reset the 'running' info field, and
clear the scan cursor, so that when it'll be re-enabled, a new fresh scan will
start.

Clearing the 'running' variable is important since lowering the defragger
tunables mid-scan won't help, the defragger only considers new threshold when
a new scan starts, and during a scan it can only become more aggressive,
(when more severe fragmentation is detected), it'll never go less aggressive.
So by temporarily disabling activedefrag, one can lower th the tunables.

Removing the experimantal warning.

src/server.c

@@ -1691,8 +1691,7 @@ void databasesCron(void) {
     }
 
     /* Defrag keys gradually. */
-    if (server.active_defrag_enabled)
-        activeDefragCycle();
+    activeDefragCycle();
 
     /* Perform hash tables rehashing if needed, but only if there are no
      * other processes saving the DB on disk. Otherwise rehashing is bad
@@ -2854,6 +2853,7 @@ void initServer(void) {
         server.db[j].id = j;
         server.db[j].avg_ttl = 0;
         server.db[j].defrag_later = listCreate();
+        listSetFreeMethod(server.db[j].defrag_later,(void (*)(void*))sdsfree);
     }
     evictionPoolAlloc(); /* Initialize the LRU keys pool. */
     server.pubsub_channels = dictCreate(&keylistDictType,NULL);