LRU: Make cross-database choices for eviction.

The LRU eviction code used to make local choices: for each DB visited it
selected the best key to evict. This was repeated for each DB. However
this means that there could be DBs with very frequently accessed keys
that are targeted by the LRU algorithm while there were other DBs with
many better candidates to expire.

This commit attempts to fix this problem for the LRU policy. However the
TTL policy is still not fixed by this commit. The TTL policy will be
fixed in a successive commit.

This is an initial (partial because of TTL policy) fix for issue #2647.

src/server.c

@@ -1748,10 +1748,10 @@ void initServer(void) {
         server.db[j].blocking_keys = dictCreate(&keylistDictType,NULL);
         server.db[j].ready_keys = dictCreate(&objectKeyPointerValueDictType,NULL);
         server.db[j].watched_keys = dictCreate(&keylistDictType,NULL);
-        server.db[j].eviction_pool = evictionPoolAlloc();
         server.db[j].id = j;
         server.db[j].avg_ttl = 0;
     }
+    evictionPoolAlloc(); /* Initialize the LRU keys pool. */
     server.pubsub_channels = dictCreate(&keylistDictType,NULL);
     server.pubsub_patterns = listCreate();
     listSetFreeMethod(server.pubsub_patterns,freePubsubPattern);