[#563] writecache: Fix metrics and bolt delete

Estimate cache size after delete objects to update metric.
Update counters on small object deletion.
Do not count bbolt DB file as FSTree object.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/writecache/writecachebbolt/flush.go

@@ -246,7 +246,7 @@ func (c *cache) workerFlushSmall() {
 			continue
 		}
 
-		c.deleteFromDB([]string{objectCore.AddressOf(obj).EncodeToString()})
+		c.deleteFromDB(objectCore.AddressOf(obj).EncodeToString())
 	}
 }