Memory now evicts bucket if no more objects in memory struct

- To avoid race in expiration while accessing memory driver structs with
  in two competing write locks. Use lru Len() instead to know exact length
  to schedule for eviction.

- squash both bucket and object structs, instead use a separate map to keep
  mutable info of lastAccessTime which can be independently used inside the
  expiration routine.

pkg/server/server.go

@@ -19,6 +19,9 @@ package server
 import (
 	"errors"
 	"fmt"
+	"reflect"
+	"time"
+
 	"github.com/minio-io/minio/pkg/api"
 	"github.com/minio-io/minio/pkg/api/web"
 	"github.com/minio-io/minio/pkg/iodine"
@@ -26,8 +29,6 @@ import (
 	"github.com/minio-io/minio/pkg/storage/drivers/donut"
 	"github.com/minio-io/minio/pkg/storage/drivers/memory"
 	"github.com/minio-io/minio/pkg/utils/log"
-	"reflect"
-	"time"
 )
 
 // MemoryFactory is used to build memory api servers