server/coordinator: fix context pitfall (#470)
server/coordinator: fix context pitfall (#470)

According to golang package initialization, we may meet some
mysterious bugs if we do not use the same context package.

That is:

```
context.Canceled != context.Canceled
```

Because the previous `Canceled` may comes from `"context"` and the
later one comes from `"golang.org/x/net/context"`.
pingcap/pd,pingcap/pd