eventdev: introduce event vector capability

eventdev: introduce event vector capability

Introduce rte_event_vector datastructure which is capable of holding
multiple uintptr_t of the same flow thereby allowing applications
to vectorize their pipeline and reducing the complexity of pipelining
the events across multiple stages.
This approach also reduces the scheduling overhead on a event device.

Add a event vector mempool create handler to create mempools based on
the best mempool ops available on a given platform.

Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Ray Kinsella <mdr@ashroe.eu>
Acked-by: Jay Jayatheerthan <jay.jayatheerthan@intel.com>