storage/config: fix example config, add warning for missing shardCount

example_config.yaml

@@ -68,7 +68,7 @@ chihaya:
 
     # The number of partitions data will be divided into in order to provide a
     # higher degree of parallelism.
-    shards: 1024
+    shard_count: 1024
 
   # This block defines configuration used for middleware executed before a
   # response has been returned to a BitTorrent client.

storage/memory/peer_store.go

@@ -63,9 +63,12 @@ func (cfg Config) LogFields() log.Fields {
 
 // New creates a new PeerStore backed by memory.
 func New(cfg Config) (storage.PeerStore, error) {
-	shardCount := 1
+	var shardCount int
 	if cfg.ShardCount > 0 {
 		shardCount = cfg.ShardCount
+	} else {
+		log.Warnln("storage: shardCount not configured, using 1 as default value.")
+		shardCount = 1
 	}
 
 	if cfg.GarbageCollectionInterval <= 0 {