fix LRU contention for first minute of uptime

Noticed in benchmarks that memcached was slow for the first minute. ~350k
gets/sec max, then it would jump up to millions/sec.

    if (it->time < current_time - ITEM_UPDATE_INTERVAL) {
current_time starts at 2, which then underflows toward 4 billion, which is
larger than it->time, so all fetches for the first minute require shuffling
the LRU. Which happens under a global lock.

This probably goofed up all short-term benchmarks, and potentially slowed down
high traffic instances which were recently restarted.

Unfortunately this means the tests no longer exercise item_update(), so some
more work is necessary.

items.c

@@ -18,13 +18,6 @@
 static void item_link_q(item *it);
 static void item_unlink_q(item *it);
 
-/*
- * We only reposition items in the LRU queue if they haven't been repositioned
- * in this many seconds. That saves us from churning on frequently-accessed
- * items.
- */
-#define ITEM_UPDATE_INTERVAL 60
-
 #define LARGEST_ID POWER_LARGEST
 typedef struct {
     uint64_t evicted;

memcached.c

@@ -188,7 +188,7 @@ static void stats_init(void) {
        did, so time(0) - time.started is never zero.  if so, things
        like 'settings.oldest_live' which act as booleans as well as
        values are now false in boolean context... */
-    process_started = time(0) - 2;
+    process_started = time(0) - ITEM_UPDATE_INTERVAL - 2;
     stats_prefix_init();
 }
 
@@ -2562,7 +2562,7 @@ static void server_stats(ADD_STAT add_stats, conn *c) {
     STATS_LOCK();
 
     APPEND_STAT("pid", "%lu", (long)pid);
-    APPEND_STAT("uptime", "%u", now);
+    APPEND_STAT("uptime", "%u", now - ITEM_UPDATE_INTERVAL);
     APPEND_STAT("time", "%ld", now + (long)process_started);
     APPEND_STAT("version", "%s", VERSION);
     APPEND_STAT("libevent", "%s", event_get_version());
@@ -4708,7 +4708,7 @@ static void clock_handler(const int fd, const short which, void *arg) {
         struct timespec ts;
         if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0) {
             monotonic = true;
-            monotonic_start = ts.tv_sec - 2;
+            monotonic_start = ts.tv_sec - ITEM_UPDATE_INTERVAL - 2;
         }
 #endif
     }

memcached.h

@@ -63,6 +63,13 @@
 /* Initial power multiplier for the hash table */
 #define HASHPOWER_DEFAULT 16
 
+/*
+ * We only reposition items in the LRU queue if they haven't been repositioned
+ * in this many seconds. That saves us from churning on frequently-accessed
+ * items.
+ */
+#define ITEM_UPDATE_INTERVAL 60
+
 /* unistd.h is here */
 #if HAVE_UNISTD_H
 # include <unistd.h>