active defrag v2

- big keys are not defragged in one go from within the dict scan
  instead they are scanned in parts after the main dict hash bucket is done.
- add latency monitor sample for defrag
- change default active-defrag-cycle-min to induce lower latency
- make active defrag start a new scan right away if needed, so it's easier
  (for the test suite) to detect when it's done
- make active defrag quick the current cycle after each db / big key
- defrag  some non key long term global allocations
- some refactoring for smaller functions and more reusable code
- during dict rehashing, one scan iteration of the dict, can end up scanning
  one bucket in the smaller dict and many many buckets in the larger dict.
  so waiting for 16 scan iterations before checking the time, may be much too long.

src/server.c

@@ -1389,6 +1389,7 @@ void initServerConfig(void) {
     server.active_defrag_threshold_upper = CONFIG_DEFAULT_DEFRAG_THRESHOLD_UPPER;
     server.active_defrag_cycle_min = CONFIG_DEFAULT_DEFRAG_CYCLE_MIN;
     server.active_defrag_cycle_max = CONFIG_DEFAULT_DEFRAG_CYCLE_MAX;
+    server.active_defrag_max_scan_fields = CONFIG_DEFAULT_DEFRAG_MAX_SCAN_FIELDS;
     server.proto_max_bulk_len = CONFIG_DEFAULT_PROTO_MAX_BULK_LEN;
     server.client_max_querybuf_len = PROTO_MAX_QUERYBUF_LEN;
     server.saveparams = NULL;
@@ -1806,6 +1807,7 @@ void resetServerStats(void) {
     server.stat_active_defrag_misses = 0;
     server.stat_active_defrag_key_hits = 0;
     server.stat_active_defrag_key_misses = 0;
+    server.stat_active_defrag_scanned = 0;
     server.stat_fork_time = 0;
     server.stat_fork_rate = 0;
     server.stat_rejected_conn = 0;
@@ -1894,6 +1896,7 @@ void initServer(void) {
         server.db[j].watched_keys = dictCreate(&keylistDictType,NULL);
         server.db[j].id = j;
         server.db[j].avg_ttl = 0;
+        server.db[j].defrag_later = listCreate();
     }
     evictionPoolAlloc(); /* Initialize the LRU keys pool. */
     server.pubsub_channels = dictCreate(&keylistDictType,NULL);