SPARK-2425 Don't kill a still-running Application because of some misbehaving Executors 

Introduces a LOADING -> RUNNING ApplicationState transition and prevents Master from removing an Application with RUNNING Executors.

Two basic changes: 1) Instead of allowing MAX_NUM_RETRY abnormal Executor exits over the entire lifetime of the Application, allow that many since any Executor successfully began running the Application; 2) Don't remove the Application while Master still thinks that there are RUNNING Executors.

This should be fine as long as the ApplicationInfo doesn't believe any Executors are forever RUNNING when they are not.  I think that any non-RUNNING Executors will eventually no longer be RUNNING in Master's accounting, but another set of eyes should confirm that.  This PR also doesn't try to detect which nodes have gone rogue or to kill off bad Workers, so repeatedly failing Executors will continue to fail and fill up log files with failure reports as long as the Application keeps running.    
That's not necessary.  If `idToApp` does not contain `appId`, then `execOption` will be `None` and we'll never end up in this `case Some(exec)`.