gc dead store

[siddontang]

If the store was down, pd should be told about it or pd should check whether the store is alive or not for a long time.

If pd ensures that the store is down, it will do balance for the region which contains the store, maybe we can do it in following cases:

• When the leader of region reports Heartbeat, pd finds the region contains the dead store and then does balance.
• Pd chooses a region which contains the dead store regularly, then does balance.

/cc @qiuyesuifeng @disksing @zhangjinpeng1987

[huachaohuang]

This issues will be separated into three steps:

• Health check for stores and filter out down stores from balance.
• Health check for regions and do balance for down peers. tikv #823
• Add admin api to remove stores gracefully.