Access 'executorDataMap' out of 'DriverEndpoint' should be protected by lock

Just as the comments:

> 

// Accessing `executorDataMap` in `DriverEndpoint.receive/receiveAndReply` doesn't need any
// protection. But accessing `executorDataMap` out of `DriverEndpoint.receive/receiveAndReply`
// must be protected by `CoarseGrainedSchedulerBackend.this`. Besides, `executorDataMap` should
// only be modified in `DriverEndpoint.receive/receiveAndReply` with protection by
// `CoarseGrainedSchedulerBackend.this`.

`executorDataMap` is not threadsafe, it should be protected by lock when accessing it out of `DriverEndpoint`