eventdev: add device stop flush callback

eventdev: add device stop flush callback

When an event device is stopped, it drains all event queues and ports.
These events may contain pointers, so to prevent memory leaks eventdev now
supports a user-provided flush callback that is called during the queue
drain process. This callback is stored in process memory, so the callback
must be registered by any process that may call rte_event_dev_stop().

This commit also clarifies the behavior of rte_event_dev_stop().

This follows this mailing list discussion:
http://dpdk.org/ml/archives/dev/2018-January/087484.html

Signed-off-by: Gage Eads <gage.eads@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>