[SPARK-10558][CORE] Fix wrong executor state in Master

`ExecutorAdded` can only be sent to `AppClient` when worker report back the executor state as `LOADING`, otherwise because of concurrency issue, `AppClient` will possibly receive `ExectuorAdded` at first, then `ExecutorStateUpdated` with `LOADING` state.

Also Master will change the executor state from `LAUNCHING` to `RUNNING` (`AppClient` report back the state as `RUNNING`), then to `LOADING` (worker report back to state as `LOADING`), it should be `LAUNCHING` -> `LOADING` -> `RUNNING`.

Also it is wrongly shown in master UI, the state of executor should be `RUNNING` rather than `LOADING`:

![screen shot 2015-09-11 at 2 30 28 pm](https://cloud.githubusercontent.com/assets/850797/9809254/3155d840-5899-11e5-8cdf-ad06fef75762.png)

Author: jerryshao <sshao@hortonworks.com>

Closes #8714 from jerryshao/SPARK-10558.