Obtain LRU clock in a resolution dependent way.

For testing purposes it is handy to have a very high resolution of the
LRU clock, so that it is possible to experiment with scripts running in
just a few seconds how the eviction algorithms works.

This commit allows Redis to use the cached LRU clock, or a value
computed on demand, depending on the resolution. So normally we have the
good performance of a precomputed value, and a clock that wraps in many
days using the normal resolution, but if needed, changing a define will
switch behavior to an high resolution LRU clock.

src/db.c

@@ -50,7 +50,7 @@ robj *lookupKey(redisDb *db, robj *key) {
          * Don't do it if we have a saving child, as this will trigger
          * a copy on write madness. */
         if (server.rdb_child_pid == -1 && server.aof_child_pid == -1)
-            val->lru = server.lruclock;
+            val->lru = LRU_CLOCK();
         return val;
     } else {
         return NULL;