some tablets were marked as ready but there are no tabletservers server them

case1

```
某个Ready的tablet，
t0时刻：开始split，状态 Ready -> OnSplit
t1时刻：不久ts挂掉，状态不变
t2时刻：master处理之前split的回调，发现是SplitNotSupport，状态 OnSplit -> Ready

此后这个tablet就处于“悬空”状态了。。
```

case2 (2016.05.30)

```
t0时刻：master发送某个tablet的unload命令后，还未收到回复，就发生了一次重启
t1时刻：ts收到unload请求后开始执行，一时半会儿没有unload掉
t2时刻：master重启时query，发现该tablet在ts上
t3时刻：ts完成unload

此后这个tablet就处于“悬空”状态了。。
```

可能还有更多类似问题，有待深究。

Start by tracing the tablet state transitions and master/tabletserver restart and unload handling described in the two cases. Reproduce or investigate how a tablet can return to Ready or finish unloading without a serving tabletserver, then define completion as preventing or recovering these orphaned tablet states.

Written by the indexing model from the issue text.