Regions won't be replicated under some conditions

Three TiKVs run on the same machine, and share the same HDD. When the HDD is almost fall, PD stop replicating regions, even if there is only one replica of those regions. It seems that `storageThresholdFilter` filters all stores if they are all almost full. I think we should guarantee enough replicas first then take case about storage threshold.

Cc @siddontang @disksing @nolouch