Can't close tasks in an event callback when using gRPC

[texasaggie97]

test___done_event_registered___call_clear_in_callback___stop_close_in_callback_with_success_status and test___every_n_samples_event_registered___call_clear_in_callback___stop_close_in_callback_with_success_status fail when closing a task in an event callback.

This is supported when not using gRPC

[zhindes]

task.close() closes all open event handlers and then GrpcEventHandler.close() joins its event thread. When calling task.close from the event itself, the current thread is the event thread. A thread joining itself doesn't work so well. I think we just need a check for that case.